Information

Do animals that sleep during the day have a different sleep architecture than those who sleep at night?

Do animals that sleep during the day have a different sleep architecture than those who sleep at night?


We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

A typical hypnogram, delineating the sleep architecture for a (human) mammal looks like

A distinct distribution of non-REM and REM sleep can be observed.

Is this pattern of sleep cycles particular to that of diurnal mammals, and if so, how would the sleep architecture of a mammal that sleeps during the daytime be different?


One study (Zhao et. al., 2010), investigating the sleep architecture of two bat species (one nocturnal, and one mixed), notes the following:

C. sphinx was found to sleep predominantly throughout the day (60% of total sleep quota) during which time it spent significantly longer time in NREM and REM sleep. Compared to E. spelaea, C. sphinx had significantly less REM and NREM sleep episodes but their duration was significantly longer. E. spelaea appears to distribute its time at wake and in REM and NREM sleep equally throughout the light and dark phases.

Top three rows are for the predominantly nocturnal bat C. sphinx. Note the "Wake" rows for the two species.

Source:

Zhao, X., Sun, H., Tang, Z., Flanders, J., Zhang, S., & Ma, Y. (2010). Characterization of the sleep architecture in two species of fruit bat. Behavioural brain research, 208(2), 497-501.


Why sleep persists is fairly easy, why it is needed is an unknown.

Sleep appears to be necessary in any organism with a brain, that is anything with any kind of concentration of neurons. That is when denied it said organisms die. So all that has to happen is that the benefits of a brain outweighs the cost of sleep.

The length of sleep needed correlates with brain size, at least the REM part of sleep other parts correlate with metabolic rates. Now this can be seemingly confounded in larger complex brains (birds and mammals especially) when organisms start folding the brain to increase neuron density without increasing overall size. In this case these organisms are increasing hte "size" of the brain without making the brain larger, by increasing density. Worse some organisms "sleep" for long periods, but only a short portion of that time involves the neural activity associated with sleep (like REM), sleep in more complex animals (those with very large complex brains) contains many functions.

In organisms with tiny brains and slow metabolisms (aka the earliest things with brains) sleep does not take very long so the cost is minimal, the benefits of a brain (and thus learning) can be high. Later as brains get larger the cost goes up but so does the benefit, if it was possible for a brain to be retained without the need for sleep it should have been selected for at this stage. So in all likelihood the need for sleep is something fundamental to how neurons function and can't be changed without seriously disruptioin their function. It is not uncommon for unfavorable things to get locked in evolutionarily in this way, the cost to change them (in this case the loss of brain function) is far larger than the cost of sleep so selection keeps it around.

Now the complexity of sleep makes sense, if you already have this required period of downtime, it makes sense evolutionarily to tack on anything else that would be best done during said time. Better to use the triggers and time for existing downtime activities for any others that get added then have even more downtime. So now we have a slew of confounding factors that muddy studies and make it hard to tell what parts are essential.

We don't know why sleep is necessary, there are many ideas but not a lot of evidence. Given the complexity of sleep this is not surprising, teasing out which functions are fundamental is not easy. There is a current leaning towards it being necessary to clear out metabolites that disrupt neural function, it appears this process is highly disruptive to the brain if the brain is awake, to the point simply shutting down brain activity (movement in particular) is far safer for the organism. But as with all research this is very preliminary and sleep is poorly understood so acceptance is and should be very tentative.


Sleeping in Short Periods Disrupts Natural Rhythms

If you suffer from inadequate rest, either of insufficient quantity or of poor quality, you are likely to experience excessive daytime sleepiness. This drowsiness can make you capable of falling asleep at almost any time. Rather than sleeping in one consolidated period of sleep overnight, you may sleep in short periods. This affects natural circadian rhythms and disrupts normal sleep cycles.

Our desire for sleep increases the longer we are awake. This is called our homeostatic sleep drive. This gradually accumulating desire for sleep builds the longer that we stay awake. We are able to resist this for many hours (even days), but eventually, the desire for sleep overwhelms us and we fall asleep. This may be due to an accumulation of neurotransmitters, chemicals in the brain that function as signals between nerve cells.

The second element that contributes to our desire for sleep is the circadian rhythm. As creatures who are typically awake during the day and asleep at night, the circadian rhythm reinforces this sleep pattern. In nocturnal animals, such as rats, the reverse pattern is seen. Various hormones in the body follow a circadian pattern. Melatonin, for example, peaks overnight. Another hormone, cortisol, plays an important role in waking us up in the morning.

These two processes come together to encourage increased drowsiness and a strong desire for sleep overnight. However, our behaviors may disrupt these natural tendencies.


Music listening near bedtime disruptive to sleep

Most people listen to music throughout their day and often near bedtime to wind down. But can that actually cause your sleep to suffer? When sleep researcher Michael Scullin, Ph.D., associate professor of psychology and neuroscience at Baylor University, realized he was waking in the middle of the night with a song stuck in his head, he saw an opportunity to study how music -- and particularly stuck songs -- might affect sleep patterns.

Scullin's recent study, published in Psychological Science, investigated the relationship between music listening and sleep, focusing on a rarely-explored mechanism: involuntary musical imagery, or "earworms," when a song or tune replays over and over in a person's mind. These commonly happen while awake, but Scullin found that they also can happen while trying to sleep.

"Our brains continue to process music even when none is playing, including apparently while we are asleep," Scullin said. "Everyone knows that music listening feels good. Adolescents and young adults routinely listen to music near bedtime. But sometimes you can have too much of a good thing. The more you listen to music, the more likely you are to catch an earworm that won't go away at bedtime. When that happens, chances are your sleep is going to suffer."

People who experience earworms regularly at night -- one or more times per week -- are six times as likely to have poor sleep quality compared to people who rarely experience earworms. Surprisingly, the study found that some instrumental music is more likely to lead to earworms and disrupt sleep quality than lyrical music.

The study involved a survey and a laboratory experiment. The survey involved 209 participants who completed a series of surveys on sleep quality, music listening habits and earworm frequency, including how often they experienced an earworm while trying to fall asleep, waking up in the middle of the night and immediately upon waking in the morning.

In the experimental study, 50 participants were brought into Scullin's Sleep Neuroscience and Cognition Laboratory at Baylor, where the research team attempted to induce earworms to determine how it affected sleep quality. Polysomnography -- a comprehensive test and the gold standard measurement for sleep -- was used to record the participants' brain waves, heart rate, breathing and more while they slept.

"Before bedtime, we played three popular and catchy songs -- Taylor Swift's 'Shake It Off,' Carly Rae Jepsen's 'Call Me Maybe' and Journey's 'Don't Stop Believin'," Scullin said. "We randomly assigned participants to listen to the original versions of those songs or the de-lyricized instrumental versions of the songs. Participants responded whether and when they experienced an earworm. Then we analyzed whether that impacted their nighttime sleep physiology. People who caught an earworm had greater difficulty falling asleep, more nighttime awakenings, and spent more time in light stages of sleep."

Additionally, EEG readings -- records of electrical activity in the brain -- from the experimental study were quantitatively analyzed to examine physiological markers of sleep-dependent memory consolidation. Memory consolidation is the process by which temporary memories are spontaneously reactived during sleep and transformed into a more long-term form.

"We thought that people would have earworms at bedtime when they were trying to fall asleep, but we certainly didn't know that people would report regularly waking up from sleep with an earworm. But we saw that in both the survey and experimental study," he said.

Participants who had a sleep earworm showed more slow oscillations during sleep, a marker of memory reactivation. The increase in slow oscillations was dominant over the region corresponding to the primary auditory cortex which is implicated in earworm processing when people are awake.

"Almost everyone thought music improves their sleep, but we found those who listened to more music slept worse," Scullin said. "What was really surprising was that instrumental music led to worse sleep quality -- instrumental music leads to about twice as many earworms."

The study found that individuals with greater music listening habits experienced persistent earworms and a decline in sleep quality. These results are contrary to the idea of music as a hypnotic that might help sleep. Health organizations commonly recommend listening to quiet music before bedtime -- recommendations that largely arise from self-reported studies. Instead, Scullin has objectively measured that the sleeping brain continues to process music for several hours, even after the music stops.

Knowing that earworms negatively affect sleep, Scullin recommends first trying to moderate music listening or taking occasional breaks if bothered by earworms. Timing of music also is important -- try to avoid it before bed.

"If you commonly pair listening to music while being in bed, then you'll have that association where being in that context might trigger an earworm even when you're not listening to music, such as when you're trying to fall asleep," he said.

Another way to get rid of an earworm is to engage in cognitive activity -- fully focusing on a task, problem or activity helps to distract your brain from earworms. Near bedtime, rather than engaging in a demanding activity or something that would disrupt your sleep, like watching TV or playing video games, Scullin suggests spending five to 10 minutes writing out a to-do list and putting thoughts to paper. A previous study by Scullin -- partially funded by a National Institutes of Health grant and the Sleep Research Society Foundation -- found that participants who took five minutes to write down upcoming tasks before bed helped "offload" those worrying thoughts about the future and led to faster sleep.


Work or sleep? Honeybee foragers opportunistically nap during the day when forage is not available

Shifts in work schedules test humans’ capacity to be flexible in the timing of both work and sleep. Honeybee, Apis mellifera, foragers also shift their work schedules, but how flexible they are in the timing of sleep as they shift the timing of work is unknown, despite the importance of colony-level plasticity in the face of a changing environment. We hypothesized that sleep schedules of foragers are not fixed and instead vary depending on the time when food is available. We trained bees to visit a food source made available for several hours in the early morning (AM) or several hours in the late afternoon (PM), then monitored their sleep behaviour for 24 h after training, specifically comparing their sleep during the AM and PM periods previously designated as training periods. Following AM training, honeybee foragers slept more during the afternoon than during the morning, but following PM training, the same bees ‘slept in’ the next morning, and so slept more in the morning than in the afternoon. Although foragers did not change the total amount of time devoted to each of their behaviours (including sleep), the timing of their sleep did change. Thus, plasticity in timing of foraging was matched by plasticity in timing of sleep. The apparent correlation between the timing patterns of foraging and sleeping demonstrates temporal plasticity of sleep under ecologically realistic conditions in an invertebrate. Testing how shift work affects the health and performance of honeybees may shed light on the role of sleep in a nonhuman social animal.


Conclusions

Disturbed sleep is a core symptom of depression and its normalization is necessary to achieve remission from the illness. In the long term, all antidepressants which show clinical efficacy improve sleep secondary to improvement of mood and daytime activity. However, in the short term, while some of them may impair sleep due to the activating effects, other may improve sleep due to the sedative properties. Although sleep-promoting action is desired in depressed patients with coexisting anxiety or insomnia, it may be problematic during the maintenance treatment after recovery from depression due to oversedation. Thus, it is necessary to understand the effects of these drugs on the sleep and daytime alertness. It is particularly noteworthy that for sleep-promoting effect, it is sufficient to use a sedative antidepressant in a low dose. In such dose, these drugs can be also combined with other antidepressants as an alternative to hypnotic drugs, especially if there is a clinical necessity to promote sleep for longer than 2𠄴 weeks with a frequency higher than 3𠄴 times per week.


Contents

Polyphasic sleep can be caused by irregular sleep-wake syndrome, a rare circadian rhythm sleep disorder which is usually caused by neurological abnormality, head injury or dementia. [3] Much more common examples are the sleep of human infants and of many animals. Elderly humans often have disturbed sleep, including polyphasic sleep. [4]

In their 2006 paper "The Nature of Spontaneous Sleep Across Adulthood", [5] Campbell and Murphy studied sleep timing and quality in young, middle-aged, and older adults. They found that, in free-running conditions, the average duration of major nighttime sleep was significantly longer in young adults than in the other groups. The paper states further:

Whether such patterns are simply a response to the relatively static experimental conditions, or whether they more accurately reflect the natural organization of the human sleep/wake system, compared with that which is exhibited in daily life, is open to debate. However, the comparative literature strongly suggests that shorter, polyphasically-placed sleep is the rule, rather than the exception, across the entire animal kingdom (Campbell and Tobler, 1984 Tobler, 1989). There is little reason to believe that the human sleep/wake system would evolve in a fundamentally different manner. That people often do not exhibit such sleep organization in daily life merely suggests that humans have the capacity (often with the aid of stimulants such as caffeine or increased physical activity) to overcome the propensity for sleep when it is desirable, or is required, to do so.

One classic cultural example of a biphasic sleep pattern is the practice of siesta, which is a nap taken in the early afternoon, often after the midday meal. Such a period of sleep is a common tradition in some countries, particularly those where the weather is warm. The siesta is historically common throughout the Mediterranean and Southern Europe. It is the traditional daytime sleep of China, [6] India, South Africa, Italy, [7] Spain and, through Spanish influence, the Philippines and many Hispanic American countries.

A separate biphasic sleep pattern is sometimes described as segmented sleep, often consisting of going to sleep early at night, awakening in the post-midnight hours, and then returning to bed for a second period of sleep into the morning. The New York Times asserts that this practice was common in the past -- "in the preindustrial West, most people slept in two discrete blocks." [8] Benjamin Franklin was a prominent example of this sleeping pattern. [8]

Interrupted sleep is a primarily biphasic sleep pattern where two periods of nighttime sleep are punctuated by a period of wakefulness. Along with a nap in the day, it has been argued that this is the natural pattern of human sleep in long winter nights. [9] [10] A case has been made that maintaining such a sleep pattern may be important in regulating stress. [10]

As historical norm Edit

Historian A. Roger Ekirch [11] [12] has argued that before the Industrial Revolution, interrupted sleep was dominant in Western civilization. He draws evidence from more than 500 references to a segmented sleeping pattern in documents from the ancient, medieval, and modern world. [10] Other historians, such as Craig Koslofsky, [13] have endorsed Ekirch's analysis.

According to Ekirch's argument, adults typically slept in two distinct phases, bridged by an intervening period of wakefulness of approximately one hour. [12] This time was used to pray [14] and reflect, [15] and to interpret dreams, which were more vivid at that hour than upon waking in the morning. This was also a favorite time for scholars and poets to write uninterrupted, whereas still others visited neighbors, engaged in sexual activity, or committed petty crime. [12] : 311–323

The human circadian rhythm regulates the human sleep-wake cycle of wakefulness during the day and sleep at night. Ekirch suggests that it is due to the modern use of electric lighting that most modern humans do not practice interrupted sleep, which is a concern for some writers. [16] Superimposed on this basic rhythm is a secondary one of light sleep in the early afternoon.

The brain exhibits high levels of the pituitary hormone prolactin during the period of nighttime wakefulness, which may contribute to the feeling of peace that many people associate with it. [17]

The modern assumption that consolidated sleep with no awakenings is the normal and correct way for human adults to sleep, may lead people to consult their doctors fearing they have maintenance insomnia or other sleep disorders. [10] If Ekirch's hypothesis is correct, their concerns might best be addressed by reassurance that their sleep conforms to historically natural sleep patterns. [18]

Ekirch has found that the two periods of night sleep were called "first sleep" (occasionally "dead sleep") and "second sleep" (or "morning sleep") in medieval England. He found that first and second sleep were also the terms in the Romance languages, as well as in the language of the Tiv of Nigeria. In French, the common term was premier sommeil or premier somme in Italian, primo sonno in Latin, primo somno or concubia nocte. [12] : 301–302 He found no common word in English for the period of wakefulness between, apart from paraphrases such as first waking or when one wakes from his first sleep and the generic watch in its old meaning of being awake. In old French an equivalent generic term is dorveille, a portmanteau of the French words dormir (to sleep) and veiller (to be awake).

Because members of modern industrialised societies, with later evening hours facilitated by electric lighting, mostly do not practice interrupted sleep, Ekirch suggests that they may have misinterpreted and mistranslated references to it in literature. Common modern interpretations of the term first sleep are "beauty sleep" and "early slumber". A reference to first sleep in the Odyssey was translated as "first sleep" in the seventeenth century, but, if Ekirch's hypothesis is correct, was universally mistranslated in the twentieth. [12] : 303

In his 1992 study "In short photoperiods, human sleep is biphasic", Thomas Wehr had seven healthy men confined to a room for fourteen hours of darkness daily for a month. At first the participants slept for about eleven hours, presumably making up for their sleep debt. After this the subjects began to sleep much as people in pre-industrial times were claimed to have done. They would sleep for about four hours, wake up for two to three hours, then go back to bed for another four hours. They also took about two hours to fall asleep. [9]

In crises and other extreme conditions, people may not be able to achieve the recommended eight hours of sleep per day. Systematic napping may be considered necessary in such situations.

Claudio Stampi, as a result of his interest in long-distance solo boat racing, has studied the systematic timing of short naps as a means of ensuring optimal performance in situations where extreme sleep deprivation is inevitable, but he does not advocate ultrashort napping as a lifestyle. [19] Scientific American Frontiers (PBS) has reported on Stampi's 49-day experiment where a young man napped for a total of three hours per day. It purportedly shows that all stages of sleep were included. [20] Stampi has written about his research in his book Why We Nap: Evolution, Chronobiology, and Functions of Polyphasic and Ultrashort Sleep (1992). [21] In 1989 he published results of a field study in the journal Work & Stress, concluding that "polyphasic sleep strategies improve prolonged sustained performance" under continuous work situations. [22] In addition, other long-distance solo sailors have documented their techniques for maximizing wake time on the open seas. One account documents the process by which a solo sailor broke his sleep into between 6 and 7 naps per day. The naps would not be placed equiphasically, instead occurring more densely during night hours. [23]

U.S. military Edit

The U.S. military has studied fatigue countermeasures. An Air Force report states:

Each individual nap should be long enough to provide at least 45 continuous minutes of sleep, although longer naps (2 hours) are better. In general, the shorter each individual nap is, the more frequent the naps should be (the objective remains to acquire a daily total of 8 hours of sleep). [24]

Canadian Marine pilots Edit

Similarly, the Canadian Marine pilots in their trainer's handbook report that:

Under extreme circumstances where sleep cannot be achieved continuously, research on napping shows that 10- to 20-minute naps at regular intervals during the day can help relieve some of the sleep deprivation and thus maintain . performance for several days. However, researchers caution that levels of performance achieved using ultrashort sleep (short naps) to temporarily replace normal sleep are always well below that achieved when fully rested. [25]

NASA Edit

NASA, in cooperation with the National Space Biomedical Research Institute, has funded research on napping. Despite NASA recommendations that astronauts sleep eight hours a day when in space, they usually have trouble sleeping eight hours at a stretch, so the agency needs to know about the optimal length, timing and effect of naps. Professor David Dinges of the University of Pennsylvania School of Medicine led research in a laboratory setting on sleep schedules which combined various amounts of "anchor sleep", ranging from about four to eight hours in length, with no nap or daily naps of up to 2.5 hours. Longer naps were found to be better, with some cognitive functions benefiting more from napping than others. Vigilance and basic alertness benefited the least while working memory benefited greatly. Naps in the individual subjects' biological daytime worked well, but naps in their nighttime were followed by much greater sleep inertia lasting up to an hour. [26]

Italian Air Force Edit

The Italian Air Force (Aeronautica Militare Italiana) also conducted experiments for their pilots. In schedules involving night shifts and fragmentation of duty periods through the entire day, a sort of polyphasic sleeping schedule was studied. Subjects were to perform two hours of activity followed by four hours of rest (sleep allowed), this was repeated four times throughout the 24-hour day. Subjects adopted a schedule of sleeping only during the final three rest periods in linearly increasing duration. The AMI published findings that "total sleep time was substantially reduced as compared to the usual 7–8 hour monophasic nocturnal sleep" while "maintaining good levels of vigilance as shown by the virtual absence of EEG microsleeps." EEG microsleeps are measurable and usually unnoticeable bursts of sleep in the brain while a subject appears to be awake. Nocturnal sleepers who sleep poorly may be heavily bombarded with microsleeps during waking hours, limiting focus and attention. [27]

There is an active community that experiments with alternative sleeping schedules to achieve more time awake each day, but the effectiveness of this is disputed. [28]

Researcher Piotr Woźniak argues that the theory behind severe reduction of total sleep time by way of short naps is unsound, and that there is no brain control mechanism that would make it possible to adapt to the "multiple naps" system. Woźniak expresses concern that the ways in which the polyphasic sleepers' attempt to limit total sleep time, restrict time spent in the various stages of the sleep cycle, and disrupt their circadian rhythms will eventually cause them to suffer the same negative effects as those with other forms of sleep deprivation or circadian rhythm sleep disorder. Woźniak claims to have scanned the blogs of polyphasic sleepers and found that they have to choose an "engaging activity" again and again just to stay awake and that polyphasic sleep does not improve one's learning ability or creativity. [29]

There are many claims [ citation needed ] that polyphasic sleep was used by polymaths and prominent people such as Leonardo da Vinci, Napoleon, and Nikola Tesla, but there are few if any reliable sources confirming these. One first person account comes from Buckminster Fuller, who described a regimen consisting of 30-minute naps every six hours. The short article about Fuller's nap schedule in Time in 1943, which referred to the schedule as "intermittent sleeping", says that he maintained it for two years, and notes that "he had to quit because his schedule conflicted with that of his business associates, who insisted on sleeping like other men." [30]


Contents

The most pronounced physiological changes in sleep occur in the brain. [8] The brain uses significantly less energy during sleep than it does when awake, especially during non-REM sleep. In areas with reduced activity, the brain restores its supply of adenosine triphosphate (ATP), the molecule used for short-term storage and transport of energy. [9] In quiet waking, the brain is responsible for 20% of the body's energy use, thus this reduction has a noticeable effect on overall energy consumption. [10]

Sleep increases the sensory threshold. In other words, sleeping persons perceive fewer stimuli, but can generally still respond to loud noises and other salient sensory events. [10] [8]

During slow-wave sleep, humans secrete bursts of growth hormone. All sleep, even during the day, is associated with secretion of prolactin. [11]

Key physiological methods for monitoring and measuring changes during sleep include electroencephalography (EEG) of brain waves, electrooculography (EOG) of eye movements, and electromyography (EMG) of skeletal muscle activity. Simultaneous collection of these measurements is called polysomnography, and can be performed in a specialized sleep laboratory. [12] [13] Sleep researchers also use simplified electrocardiography (EKG) for cardiac activity and actigraphy for motor movements. [13]

Non-REM and REM sleep

Sleep is divided into two broad types: non-rapid eye movement (non-REM or NREM) sleep and rapid eye movement (REM) sleep. Non-REM and REM sleep are so different that physiologists identify them as distinct behavioral states. Non-REM sleep occurs first and after a transitional period is called slow-wave sleep or deep sleep. During this phase, body temperature and heart rate fall, and the brain uses less energy. [8] REM sleep, also known as paradoxical sleep, represents a smaller portion of total sleep time. It is the main occasion for dreams (or nightmares), and is associated with desynchronized and fast brain waves, eye movements, loss of muscle tone, [2] and suspension of homeostasis. [14]

The sleep cycle of alternate NREM and REM sleep takes an average of 90 minutes, occurring 4–6 times in a good night's sleep. [13] [15] The American Academy of Sleep Medicine (AASM) divides NREM into three stages: N1, N2, and N3, the last of which is also called delta sleep or slow-wave sleep. [16] The whole period normally proceeds in the order: N1 → N2 → N3 → N2 → REM. REM sleep occurs as a person returns to stage 2 or 1 from a deep sleep. [2] There is a greater amount of deep sleep (stage N3) earlier in the night, while the proportion of REM sleep increases in the two cycles just before natural awakening. [13]

Awakening

Awakening can mean the end of sleep, or simply a moment to survey the environment and readjust body position before falling back asleep. Sleepers typically awaken soon after the end of a REM phase or sometimes in the middle of REM. Internal circadian indicators, along with a successful reduction of homeostatic sleep need, typically bring about awakening and the end of the sleep cycle. [17] Awakening involves heightened electrical activation in the brain, beginning with the thalamus and spreading throughout the cortex. [17]

During a night's sleep, a small amount of time is usually spent in a waking state. As measured by electroencephalography, young females are awake for 0–1% of the larger sleeping period young males are awake for 0–2%. In adults, wakefulness increases, especially in later cycles. One study found 3% awake time in the first ninety-minute sleep cycle, 8% in the second, 10% in the third, 12% in the fourth, and 13–14% in the fifth. Most of this awake time occurred shortly after REM sleep. [17]

Today, many humans wake up with an alarm clock [18] however, people can also reliably wake themselves up at a specific time with no need for an alarm. [17] Many sleep quite differently on workdays versus days off, a pattern which can lead to chronic circadian desynchronization. [19] [18] Many people regularly look at television and other screens before going to bed, a factor which may exacerbate disruption of the circadian cycle. [20] [21] Scientific studies on sleep have shown that sleep stage at awakening is an important factor in amplifying sleep inertia. [22]

Sleep timing is controlled by the circadian clock (Process C), sleep-wake homeostasis (Process S), and to some extent by the individual will.

Circadian clock

Sleep timing depends greatly on hormonal signals from the circadian clock, or Process C, a complex neurochemical system which uses signals from an organism's environment to recreate an internal day–night rhythm. Process C counteracts the homeostatic drive for sleep during the day (in diurnal animals) and augments it at night. [23] [19] The suprachiasmatic nucleus (SCN), a brain area directly above the optic chiasm, is presently considered the most important nexus for this process however, secondary clock systems have been found throughout the body.

An organism whose circadian clock exhibits a regular rhythm corresponding to outside signals is said to be entrained an entrained rhythm persists even if the outside signals suddenly disappear. If an entrained human is isolated in a bunker with constant light or darkness, he or she will continue to experience rhythmic increases and decreases of body temperature and melatonin, on a period that slightly exceeds 24 hours. Scientists refer to such conditions as free-running of the circadian rhythm. Under natural conditions, light signals regularly adjust this period downward, so that it corresponds better with the exact 24 hours of an Earth day. [18] [24] [25]

The circadian clock exerts constant influence on the body, affecting sinusoidal oscillation of body temperature between roughly 36.2 °C and 37.2 °C. [25] [26] The suprachiasmatic nucleus itself shows conspicuous oscillation activity, which intensifies during subjective day (i.e., the part of the rhythm corresponding with daytime, whether accurately or not) and drops to almost nothing during subjective night. [27] The circadian pacemaker in the suprachiasmatic nucleus has a direct neural connection to the pineal gland, which releases the hormone melatonin at night. [27] Cortisol levels typically rise throughout the night, peak in the awakening hours, and diminish during the day. [11] [28] Circadian prolactin secretion begins in the late afternoon, especially in women, and is subsequently augmented by sleep-induced secretion, to peak in the middle of the night. Circadian rhythm exerts some influence on the nighttime secretion of growth hormone. [11]

The circadian rhythm influences the ideal timing of a restorative sleep episode. [18] [29] Sleepiness increases during the night. REM sleep occurs more during body temperature minimum within the circadian cycle, whereas slow-wave sleep can occur more independently of circadian time. [25]

The internal circadian clock is profoundly influenced by changes in light, since these are its main clues about what time it is. Exposure to even small amounts of light during the night can suppress melatonin secretion, and increase body temperature and wakefulness. Short pulses of light, at the right moment in the circadian cycle, can significantly 'reset' the internal clock. [26] Blue light, in particular, exerts the strongest effect, [19] leading to concerns that electronic media use before bed may interfere with sleep. [20]

Modern humans often find themselves desynchronized from their internal circadian clock, due to the requirements of work (especially night shifts), long-distance travel, and the influence of universal indoor lighting. [25] Even if they have sleep debt, or feel sleepy, people can have difficulty staying asleep at the peak of their circadian cycle. Conversely, they can have difficulty waking up in the trough of the cycle. [17] A healthy young adult entrained to the sun will (during most of the year) fall asleep a few hours after sunset, experience body temperature minimum at 6 a.m., and wake up a few hours after sunrise. [25]

Process S

Generally speaking, the longer an organism is awake, the more it feels a need to sleep ("sleep debt"). This driver of sleep is referred to as Process S. The balance between sleeping and waking is regulated by a process called homeostasis. Induced or perceived lack of sleep is called sleep deprivation.

Process S is driven by the depletion of glycogen and accumulation of adenosine in the forebrain that disinhibits the ventrolateral preoptic nucleus, allowing for inhibition of the ascending reticular activating system. [30]

Sleep deprivation tends to cause slower brain waves in the frontal cortex, shortened attention span, higher anxiety, impaired memory, and a grouchy mood. Conversely, a well-rested organism tends to have improved memory and mood. [31] Neurophysiological and functional imaging studies have demonstrated that frontal regions of the brain are particularly responsive to homeostatic sleep pressure. [32]

There is disagreement on how much sleep debt is possible to accumulate, and whether sleep debt is accumulated against an individual's average sleep or some other benchmark. It is also unclear whether the prevalence of sleep debt among adults has changed appreciably in the industrialized world in recent decades. Sleep debt does show some evidence of being cumulative. Subjectively, however, humans seem to reach maximum sleepiness after 30 hours of waking up. [25] It is likely that in Western societies, children are sleeping less than they previously have. [33]

One neurochemical indicator of sleep debt is adenosine, a neurotransmitter that inhibits many of the bodily processes associated with wakefulness. Adenosine levels increase in the cortex and basal forebrain during prolonged wakefulness, and decrease during the sleep-recovery period, potentially acting as a homeostatic regulator of sleep. [34] [35] Coffee and caffeine temporarily block the effect of adenosine, prolong sleep latency, and reduce total sleep time and quality. [36]

Social timing

Humans are also influenced by aspects of social time, such as the hours when other people are awake, the hours when work is required, the time on the clock, etc. Time zones, standard times used to unify the timing for people in the same area, correspond only approximately to the natural rising and setting of the sun. The approximate nature of the time zone can be shown with China, a country which used to span five time zones and now officially uses only one (UTC+8). [18]

Distribution

In polyphasic sleep, an organism sleeps several times in a 24-hour cycle, whereas in monophasic sleep this occurs all at once. Under experimental conditions, humans tend to alternate more frequently between sleep and wakefulness (i.e., exhibit more polyphasic sleep) if they have nothing better to do. [25] Given a 14-hour period of darkness in experimental conditions, humans tended towards bimodal sleep, with two sleep periods concentrated at the beginning and at the end of the dark time. Bimodal sleep in humans was more common before the industrial revolution. [28]

Different characteristic sleep patterns, such as the familiarly so-called "early bird" and "night owl", are called chronotypes. Genetics and sex have some influence on chronotype, but so do habits. Chronotype is also liable to change over the course of a person's lifetime. Seven-year-olds are better disposed to wake up early in the morning than are fifteen-year-olds. [19] [18] Chronotypes far outside the normal range are called circadian rhythm sleep disorders. [37]

The siesta habit has recently been associated with a 37% lower coronary mortality, possibly due to reduced cardiovascular stress mediated by daytime sleep. [38] Short naps at mid-day and mild evening exercise were found to be effective for improved sleep, cognitive tasks, and mental health in elderly people. [39]

Genetics

Monozygotic (identical) but not dizygotic (fraternal) twins tend to have similar sleep habits. Neurotransmitters, molecules whose production can be traced to specific genes, are one genetic influence on sleep that can be analyzed. The circadian clock has its own set of genes. [40] Genes which may influence sleep include ABCC9, DEC2, Dopamine receptor D2 [41] and variants near PAX 8 and VRK2. [42]

Quality

The quality of sleep may be evaluated from an objective and a subjective point of view. Objective sleep quality refers to how difficult it is for a person to fall asleep and remain in a sleeping state, and how many times they wake up during a single night. Poor sleep quality disrupts the cycle of transition between the different stages of sleep. [43] Subjective sleep quality in turn refers to a sense of being rested and regenerated after awaking from sleep. A study by A. Harvey et al. (2002) found that insomniacs were more demanding in their evaluations of sleep quality than individuals who had no sleep problems. [44]

Homeostatic sleep propensity (the need for sleep as a function of the amount of time elapsed since the last adequate sleep episode) must be balanced against the circadian element for satisfactory sleep. [45] [46] Along with corresponding messages from the circadian clock, this tells the body it needs to sleep. [47] The timing is correct when the following two circadian markers occur after the middle of the sleep episode and before awakening: [48] maximum concentration of the hormone melatonin, and minimum core body temperature.

Human sleep-needs vary by age and amongst individuals sleep is considered to be adequate when there is no daytime sleepiness or dysfunction. Moreover, self-reported sleep duration is only moderately correlated with actual sleep time as measured by actigraphy, [50] and those affected with sleep state misperception may typically report having slept only four hours despite having slept a full eight hours. [51]

Researchers have found that sleeping 6–7 hours each night correlates with longevity and cardiac health in humans, though many underlying factors may be involved in the causality behind this relationship. [52] [53] [54] [55] [42] [56] [57]

Sleep difficulties are furthermore associated with psychiatric disorders such as depression, alcoholism, and bipolar disorder. [58] Up to 90 percent of adults with depression are found to have sleep difficulties. Dysregulation detected by EEG includes disturbances in sleep continuity, decreased delta sleep and altered REM patterns with regard to latency, distribution across the night and density of eye movements. [59]

Sleep duration can also vary according to season. Up to 90% of people report longer sleep duration in winter, which may lead to more pronounced seasonal affective disorder. [60] [61]

Children

By the time infants reach the age of two, their brain size has reached 90 percent of an adult-sized brain [62] a majority of this brain growth has occurred during the period of life with the highest rate of sleep. The hours that children spend asleep influence their ability to perform on cognitive tasks. [63] [64] Children who sleep through the night and have few night waking episodes have higher cognitive attainments and easier temperaments than other children. [64] [65] [66]

Sleep also influences language development. To test this, researchers taught infants a faux language and observed their recollection of the rules for that language. [67] Infants who slept within four hours of learning the language could remember the language rules better, while infants who stayed awake longer did not recall those rules as well. There is also a relationship between infants' vocabulary and sleeping: infants who sleep longer at night at 12 months have better vocabularies at 26 months. [66]

Recommendations

Children need many hours of sleep per day in order to develop and function properly: up to 18 hours for newborn babies, with a declining rate as a child ages. [47] Early in 2015, after a two-year study, [68] the National Sleep Foundation in the US announced newly-revised recommendations as shown in the table below.

Hours of sleep required for each age group [68]
Age and condition Sleep needs
Newborns (0–3 months) 14 to 17 hours
Infants (4–11 months) 12 to 15 hours
Toddlers (1–2 years) 11 to 14 hours
Preschoolers (3–4 years) 10 to 13 hours
School-age children (5–12 years) 9 to 11 hours
Teenagers (13–17 years) 8 to 10 hours
Adults (18–64 years) 7 to 9 hours
Older Adults (65 years and over) 7 to 8 hours

Restoration

The human organism physically restores itself during sleep, occurring mostly during slow-wave sleep during which body temperature, heart rate, and brain oxygen consumption decrease. In both the brain and body, the reduced rate of metabolism enables countervailing restorative processes. [69] The brain requires sleep for restoration, whereas these processes can take place during quiescent waking in the rest of the body. [ citation needed ] The essential function of sleep may be its restorative effect on the brain: "Sleep is of the brain, by the brain and for the brain." [70] This theory is strengthened by the fact that sleep is observed to be a necessary behavior across most of the animal kingdom, including some of the least evolved animals which have no need for other functions of sleep, such as memory consolidation or dreaming. [6]

While awake, brain metabolism generates end products, such as reactive oxygen species, which may be damaging to brain cells and inhibit their proper function. During sleep, metabolic rates decrease and reactive oxygen species generation is reduced, enabling restorative processes. The sleeping brain has been shown to remove metabolic end products at a faster rate than during an awake state. [ citation needed ] The mechanism for this removal appears to be the glymphatic system. [71] Sleep may facilitate the synthesis of molecules that help repair and protect the brain from metabolic end products generated during waking. [72] Anabolic hormones, such as growth hormones, are secreted preferentially during sleep. The brain concentration of glycogen increases during sleep, and is depleted through metabolism during wakefulness. [69]

The effect of sleep duration on somatic growth is not completely known. One study recorded growth, height, and weight, as correlated to parent-reported time in bed in 305 children over a period of nine years (age 1–10). It was found that "the variation of sleep duration among children does not seem to have an effect on growth." [73] Slow-wave sleep affects growth hormone levels in adult men. [11] During eight hours of sleep, men with a high percentage of slow-wave sleep (SWS) (average 24%) also had high growth hormone secretion, while subjects with a low percentage of SWS (average 9%) had low growth hormone secretion. [74]

Memory processing

It has been widely accepted that sleep must support the formation of long-term memory, and generally increasing previous learning and experiences recalls. However, its benefit seems to depend on the phase of sleep and the type of memory. [75] For example, declarative and procedural memory-recall tasks applied over early and late nocturnal sleep, as well as wakefulness controlled conditions, have been shown that declarative memory improves more during early sleep (dominated by SWS) while procedural memory during late sleep (dominated by REM sleep) does so. [76] [77]

With regard to declarative memory, the functional role of SWS has been associated with hippocampal replays of previously encoded neural patterns that seem to facilitate long-term memory consolidation. [76] [77] This assumption is based on the active system consolidation hypothesis, which states that repeated reactivations of newly-encoded information in the hippocampus during slow oscillations in NREM sleep mediate the stabilization and gradual integration of declarative memory with pre-existing knowledge networks on the cortical level. [78] It assumes the hippocampus might hold information only temporarily and in a fast-learning rate, whereas the neocortex is related to long-term storage and a slow-learning rate. [76] [77] [79] [80] [81] This dialogue between the hippocampus and neocortex occurs in parallel with hippocampal sharp-wave ripples and thalamo-cortical spindles, synchrony that drives the formation of the spindle-ripple event which seems to be a prerequisite for the formation of long-term memories. [77] [79] [81] [82]

Reactivation of memory also occurs during wakefulness and its function is associated with serving to update the reactivated memory with newly-encoded information, whereas reactivations during SWS are presented as crucial for memory stabilization. [77] Based on targeted memory reactivation (TMR) experiments that use associated memory cues to triggering memory traces during sleep, several studies have been reassuring the importance of nocturnal reactivations for the formation of persistent memories in neocortical networks, as well as highlighting the possibility of increasing people’s memory performance at declarative recalls. [76] [80] [81] [82] [83]

Furthermore, nocturnal reactivation seems to share the same neural oscillatory patterns as reactivation during wakefulness, processes which might be coordinated by theta activity. [84] During wakefulness, theta oscillations have been often related to successful performance in memory tasks, and cued memory reactivations during sleep have been showing that theta activity is significantly stronger in subsequent recognition of cued stimuli as compared to uncued ones, possibly indicating a strengthening of memory traces and lexical integration by cuing during sleep. [85] However, the beneficial effect of TMR for memory consolidation seems to occur only if the cued memories can be related to prior knowledge. [86]

Dreaming

During sleep, especially REM sleep, humans tend to experience dreams. These are elusive and mostly unpredictable first-person experiences which seem logical and realistic to the dreamer while they are in progress, despite their frequently bizarre, irrational, and/or surreal qualities that become apparent when assessed after waking. Dreams often seamlessly incorporate concepts, situations, people, and objects within a person's mind that would not normally go together. They can include apparent sensations of all types, especially vision and movement. [87]

Dreams tend to rapidly fade from memory after waking. Some people choose to keep a dream journal, which they believe helps them build dream recall and facilitate the ability to experience lucid dreams.

People have proposed many hypotheses about the functions of dreaming. Sigmund Freud postulated that dreams are the symbolic expression of frustrated desires that have been relegated to the unconscious mind, and he used dream interpretation in the form of psychoanalysis in attempting to uncover these desires. [88]

Counterintuitively, penile erections during sleep are not more frequent during sexual dreams than during other dreams. [89] The parasympathetic nervous system experiences increased activity during REM sleep which may cause erection of the penis or clitoris. In males, 80% to 95% of REM sleep is normally accompanied by partial to full penile erection, while only about 12% of men's dreams contain sexual content. [90]

John Allan Hobson and Robert McCarley propose that dreams are caused by the random firing of neurons in the cerebral cortex during the REM period. Neatly, this theory helps explain the irrationality of the mind during REM periods, as, according to this theory, the forebrain then creates a story in an attempt to reconcile and make sense of the nonsensical sensory information presented to it. This would explain the odd nature of many dreams. [91]

Using antidepressants, [ clarification needed ] acetaminophen, ibuprofen, or alcoholic beverages is thought to potentially suppress dreams, whereas melatonin may have the ability to encourage them. [92]

Insomnia

Insomnia is a general term for difficulty falling asleep and/or staying asleep. Insomnia is the most common sleep problem, with many adults reporting occasional insomnia, and 10–15% reporting a chronic condition. [93] Insomnia can have many different causes, including psychological stress, a poor sleep environment, an inconsistent sleep schedule, or excessive mental or physical stimulation in the hours before bedtime. Insomnia is often treated through behavioral changes like keeping a regular sleep schedule, avoiding stimulating or stressful activities before bedtime, and cutting down on stimulants such as caffeine. The sleep environment may be improved by installing heavy drapes to shut out all sunlight, and keeping computers, televisions, and work materials out of the sleeping area.

A 2010 review of published scientific research suggested that exercise generally improves sleep for most people, and helps sleep disorders such as insomnia. The optimum time to exercise may be 4 to 8 hours before bedtime, though exercise at any time of day is beneficial, with the exception of heavy exercise taken shortly before bedtime, which may disturb sleep. However, there is insufficient evidence to draw detailed conclusions about the relationship between exercise and sleep. [94] Sleeping medications such as Ambien and Lunesta are an increasingly popular treatment for insomnia. Although these nonbenzodiazepine medications are generally believed to be better and safer than earlier generations of sedatives, they have still generated some controversy and discussion regarding side effects. White noise appears to be a promising treatment for insomnia. [95]

Obstructive sleep apnea

Obstructive sleep apnea is a condition in which major pauses in breathing occur during sleep, disrupting the normal progression of sleep and often causing other more severe health problems. Apneas occur when the muscles around the patient's airway relax during sleep, causing the airway to collapse and block the intake of oxygen. [96] Obstructive sleep apnea is more common than central sleep apnea. [97] As oxygen levels in the blood drop, the patient then comes out of deep sleep in order to resume breathing. When several of these episodes occur per hour, sleep apnea rises to a level of seriousness that may require treatment.

Diagnosing sleep apnea usually requires a professional sleep study performed in a sleep clinic, because the episodes of wakefulness caused by the disorder are extremely brief and patients usually do not remember experiencing them. Instead, many patients simply feel tired after getting several hours of sleep and have no idea why. Major risk factors for sleep apnea include chronic fatigue, old age, obesity, and snoring.

Aging and sleep

People over age 60 with prolonged sleep (8-10 hours or more average sleep duration of 7-8 hours in the elderly) have a 33% increased risk of all-cause mortality and 43% increased risk of cardiovascular diseases, while those with short sleep (less than 7 hours) have a 6% increased risk of all-cause mortality. [98] Sleep disorders, including sleep apnea, insomnia, or periodic limb movements, occur more commonly in the elderly, each possibly impacting sleep quality and duration. [98] A 2017 review indicated that older adults do not need less sleep, but rather have an impaired ability to obtain their sleep needs, and may be able to deal with sleepiness better than younger adults. [99] Various practices are recommended to mitigate sleep disturbances in the elderly, such as having a light bedtime snack, avoidance of caffeine, daytime naps, excessive evening stimulation, and tobacco products, and using regular bedtime and wake schedules. [100]

Other disorders

Sleep disorders include narcolepsy, periodic limb movement disorder (PLMD), restless leg syndrome (RLS), upper airway resistance syndrome (UARS), and the circadian rhythm sleep disorders. Fatal familial insomnia, or FFI, an extremely rare genetic disease with no known treatment or cure, is characterized by increasing insomnia as one of its symptoms ultimately sufferers of the disease stop sleeping entirely, before dying of the disease. [101]

Somnambulism, known as sleepwalking, is a sleeping disorder, especially among children. [102]

Low quality sleep has been linked with health conditions like cardiovascular disease, obesity, and mental illness. While poor sleep is common among those with cardiovascular disease, some research indicates that poor sleep can be a contributing cause. Short sleep duration of less than seven hours is correlated with coronary heart disease and increased risk of death from coronary heart disease. Sleep duration greater than nine hours is also correlated with coronary heart disease, as well as stroke and cardiovascular events. [103]

In both children and adults, short sleep duration is associated with an increased risk of obesity, with various studies reporting an increased risk of 45–55%. Other aspects of sleep health have been associated with obesity, including daytime napping, sleep timing, the variability of sleep timing, and low sleep efficiency. However, sleep duration is the most-studied for its impact on obesity. [103]

Sleep problems have been frequently viewed as a symptom of mental illness rather than a causative factor. However, a growing body of evidence suggests that they are both a cause and a symptom of mental illness. Insomnia is a significant predictor of major depressive disorder a meta-analysis of 170,000 people showed that insomnia at the beginning of a study period indicated a more than the twofold increased risk for major depressive disorder. Some studies have also indicated correlation between insomnia and anxiety, post-traumatic stress disorder, and suicide. Sleep disorders can increase the risk of psychosis and worsen the severity of psychotic episodes. [103]

Drugs which induce sleep, known as hypnotics, include benzodiazepines, although these interfere with REM [104] Nonbenzodiazepine hypnotics such as eszopiclone (Lunesta), zaleplon (Sonata), and zolpidem (Ambien) Antihistamines, such as diphenhydramine (Benadryl) and doxylamine Alcohol (ethanol), despite its rebound effect later in the night and interference with REM [104] [105] barbiturates, which have the same problem melatonin, a component of the circadian clock, and released naturally at night by the pineal gland [106] and cannabis, which may also interfere with REM. [107]

Stimulants, which inhibit sleep, include caffeine, an adenosine antagonist amphetamine, MDMA, empathogen-entactogens, and related drugs cocaine, which can alter the circadian rhythm, [108] [109] and methylphenidate, which acts similarly and other analeptic drugs like modafinil and armodafinil with poorly understood mechanisms.

Dietary and nutritional choices may affect sleep duration and quality. One 2016 review indicated that a high-carbohydrate diet promoted a shorter onset to sleep and a longer duration of sleep than a high-fat diet. [110] A 2012 investigation indicated that mixed micronutrients and macronutrients are needed to promote quality sleep. [111] A varied diet containing fresh fruits and vegetables, low saturated fat, and whole grains may be optimal for individuals seeking to improve sleep quality. [110] High-quality clinical trials on long-term dietary practices are needed to better define the influence of diet on sleep quality. [110]

Anthropology

Research suggests that sleep patterns vary significantly across cultures. [112] [113] The most striking differences are observed between societies that have plentiful sources of artificial light and ones that do not. [112] The primary difference appears to be that pre-light cultures have more broken-up sleep patterns. [112] For example, people without artificial light might go to sleep far sooner after the sun sets, but then wake up several times throughout the night, punctuating their sleep with periods of wakefulness, perhaps lasting several hours. [112]

The boundaries between sleeping and waking are blurred in these societies. [112] Some observers believe that nighttime sleep in these societies is most often split into two main periods, the first characterized primarily by deep sleep and the second by REM sleep. [112]

Some societies display a fragmented sleep pattern in which people sleep at all times of the day and night for shorter periods. In many nomadic or hunter-gatherer societies, people will sleep on and off throughout the day or night depending on what is happening. [112] Plentiful artificial light has been available in the industrialized West since at least the mid-19th century, and sleep patterns have changed significantly everywhere that lighting has been introduced. [112] In general, people sleep in a more concentrated burst through the night, going to sleep much later, although this is not always the case. [112]

Historian A. Roger Ekirch thinks that the traditional pattern of "segmented sleep," as it is called, began to disappear among the urban upper class in Europe in the late 17th century and the change spread over the next 200 years by the 1920s "the idea of a first and second sleep had receded entirely from our social consciousness." [114] [115] Ekirch attributes the change to increases in "street lighting, domestic lighting and a surge in coffee houses," which slowly made nighttime a legitimate time for activity, decreasing the time available for rest. [115] Today in most societies people sleep during the night, but in very hot climates they may sleep during the day. [116] During Ramadan, many Muslims sleep during the day rather than at night. [117]

In some societies, people sleep with at least one other person (sometimes many) or with animals. In other cultures, people rarely sleep with anyone except for an intimate partner. In almost all societies, sleeping partners are strongly regulated by social standards. For example, a person might only sleep with the immediate family, the extended family, a spouse or romantic partner, children, children of a certain age, children of a specific gender, peers of a certain gender, friends, peers of equal social rank, or with no one at all. Sleep may be an actively social time, depending on the sleep groupings, with no constraints on noise or activity. [112]

People sleep in a variety of locations. Some sleep directly on the ground others on a skin or blanket others sleep on platforms or beds. Some sleep with blankets, some with pillows, some with simple headrests, some with no head support. These choices are shaped by a variety of factors, such as climate, protection from predators, housing type, technology, personal preference, and the incidence of pests. [112]

In mythology and literature

Sleep has been seen in culture as similar to death since antiquity [118] in Greek mythology, Hypnos (the god of sleep) and Thanatos (the god of death) were both said to be the children of Nyx (the goddess of night). [118] John Donne, Samuel Taylor Coleridge, Percy Bysshe Shelley, and other poets have all written poems about the relationship between sleep and death. [118] Shelley describes them as "both so passing, strange and wonderful!" [118] Many people consider dying in one's sleep the most peaceful way to die. [118] Phrases such as "big sleep" and "rest in peace" are often used in reference to death, [118] possibly in an effort to lessen its finality. [118] Sleep and dreaming have sometimes been seen as providing the potential for visionary experiences. In medieval Irish tradition, in order to become a filí, the poet was required to undergo a ritual called the imbas forosnai, in which they would enter a mantic, trancelike sleep. [119] [120]

Many cultural stories have been told about people falling asleep for extended periods of time. [121] [122] The earliest of these stories is the ancient Greek legend of Epimenides of Knossos. [121] [123] [124] [125] According to the biographer Diogenes Laërtius, Epimenides was a shepherd on the Greek island of Crete. [121] [126] One day, one of his sheep went missing and he went out to look for it, but became tired and fell asleep in a cave under Mount Ida. [121] [126] When he awoke, he continued searching for the sheep, but could not find it, [121] [126] so he returned to his old farm, only to discover that it was now under new ownership. [121] [126] He went to his hometown, but discovered that nobody there knew him. [121] Finally, he met his younger brother, who was now an old man, [121] [126] and learned that he had been asleep in the cave for fifty-seven years. [121] [126]

A far more famous instance of a "long sleep" today is the Christian legend of the Seven Sleepers of Ephesus, [121] in which seven Christians flee into a cave during pagan times in order to escape persecution, [121] but fall asleep and wake up 360 years later to discover, to their astonishment, that the Roman Empire is now predominantly Christian. [121] The American author Washington Irving's short story "Rip Van Winkle", first published in 1819 in his collection of short stories The Sketch Book of Geoffrey Crayon, Gent., [122] [127] is about a man in colonial America named Rip Van Winkle who falls asleep on one of the Catskill Mountains and wakes up twenty years later after the American Revolution. [122] The story is now considered one of the greatest classics of American literature. [122]

In art

Of the thematic representations of sleep in art, physician and sleep researcher Meir Kryger wrote, "[Artists] have intense fascination with mythology, dreams, religious themes, the parallel between sleep and death, reward, abandonment of conscious control, healing, a depiction of innocence and serenity, and the erotic." [128]


Methods

Participants

Participants (n = 112) comprised adolescents in the Need for Sleep (NFS) 4 15,28 and NFS5 44 studies. They were between 15 and 19 years old, had no history of chronic medical, psychiatric or sleep disorders, a body mass index (BMI) of ≤ 30 kg/m 2 , were not habitually short sleepers (actigraphically assessed TIB ≥ 6 h with weekend sleep extension ≤ 1 h), consumed ≤ 5 caffeinated beverages per day, and did not travel across > 2 time zones one month prior to the study.

Participants in both studies were randomised into continuous and split sleep groups with total sleep opportunity differing between the two studies (6.5 h in NFS4 and 8 h in NFS5), resulting in four groups: 6.5 h-continuous (n = 29), 6.5 h-split (n = 29), 8 h-continuous (n = 29) and 8 h-split (n = 24). Those in the continuous groups were afforded an opportunity to sleep only at night, while those in the split sleep groups had a night sleep opportunity and a 1.5-h afternoon nap. Both studies were registered clinical trials (NCT03333512 and NCT04044885).

Groups did not significantly differ in age, sex, BMI, daily caffeine consumption, morningness-eveningness preference (Morningness Eveningness Questionnaire), levels of excessive daytime sleepiness (Epworth Sleepiness Scale), symptoms of chronic sleep reduction (Chronic Sleep Reduction Questionnaire), self-reported sleep quality (Pittsburgh Sleep Quality Index) and actigraphically assessed sleep patterns (p > 0.22 Table 1). Informed consent was obtained from all participants and legal guardians in compliance with a protocol approved by the National University of Singapore Institutional Review Board. All experiments were performed in accordance with relevant guidelines and regulations. Participants were compensated financially upon completion of the study.

Procedure

The NFS4 and NFS5 study protocols tracked cognitive performance across 15 days (Fig. 1) as part of a summer camp at a local boarding school. Participants first attended a briefing at least two weeks prior to commencement of the study where an actiwatch was provided for a one week period in order to assess habitual sleep (Table 1). Subsequently, participants also wore an actiwatch in the one week period immediately prior to commencement of the summer camp protocols. This confirmed that all participants refrained from napping and adhered to a 9 h sleep schedule (23:00–08:00), and were therefore well-rested at the beginning of the camp.

The protocol was conducted in a boarding school. Both protocols began with two baseline nights of 9 h TIB followed by two cycles of sleep manipulation and recovery (9 h TIB). The first cycle consisted of five nights of manipulation (M11–M15) followed by two nights of recovery (R11–R12), while the second cycle involved three nights of manipulation (M21–M23) followed by two nights of recovery (R21–R22). The second week was shorter purely for logistic reasons. On manipulation nights, the 6.5 h-continuous schedule included 6.5 h nocturnal TIB (00:15–06:45), while the 6.5 h-split schedule included 5 h nocturnal TIB (01:00–06:00) followed by a 1.5 h nap opportunity (14:00–15:30). The 8 h-continuous schedule included 8 h nocturnal TIB (23:30–07:30) while the 8 h-split schedule included 6.5 h nocturnal TIB (00:15–06:45) and a 1.5 h daytime nap opportunity (14:00–15:30). Thus, nocturnal sleep episodes were anchored to mid-sleep at 03:30 for all sleep schedules. This provided bedtimes and wake times that were practicable, while minimizing the confounding influence of evening or morning light exposure on circadian phase. Sleep–wake patterns were monitored throughout the protocol using actigraphy. Polysomnography (PSG) was recorded on nine nights (B1, B2, M11, M13, M15, R11, M21, M23, and R21), and daytime naps were monitored with PSG on five manipulation days (M11, M13, M15, M21, and M23). Split and continuous groups were housed on different floors. All participants slept in twin-share, air-conditioned rooms. Participants were allowed to adjust their room temperature according to their own comfort. Bedroom windows were fitted with blackout panels to prevent participants from being awoken by sunlight.

All cognitive tasks were programmed in E-Prime 2.0 (Psychology Software Tools, Inc., Sharpsburg, PA) and were administered via individual laptops in a classroom setting. Participants were required to wear earphones to minimize distraction. To avoid circadian confounds, timing of tasks was identical for all groups. Picture encoding took place on M15, and retrieval after two recovery nights on R12, both at 16:45. Learning sessions for the factual knowledge task took place during the second week of sleep manipulation (M21–M23) with the morning session at 11:00 and the afternoon session at 16:45. The factual knowledge retrieval took place after one recovery night on R21 at 20:30. This evening test ensured that the circadian context during retrieval would not favour retrieval of materials learned in either the morning or afternoon. The difference in length of recovery periods between learning and testing of the two memory tasks was for camp scheduling reasons. There is little evidence that sleep deprivation impairs retrieval processes themselves 8 , therefore a one night recovery period was deemed to be sufficient.

Apart from cognitive testing periods, participants spent their time in a common room that received natural as well as electrical lighting. During free periods, participants were allowed to play board games, read, and use their own electronic devices. They were allowed to interact freely with research staff and the other participants. Strenuous physical activities and the consumption of caffeine were prohibited. Activities and cognitive tasks were similar across NFS studies. No additional memory tasks occurred in the period between learning and test sessions for our two reported memory tasks in order to minimise interference effects. All participants were constantly monitored by the research staff.

Picture-encoding task

The task consisted of 240 coloured scene images presented centrally on a computer screen one at a time. Half of the images depicted scenes containing buildings while the remaining half depicted landscapes with no buildings. The images were split into 3 sets of 80 images (40 buildings and 40 no-buildings). Two sets (160 images) were presented during both the encoding and retrieval sessions while the last remaining set (80 images) served as foil images to be presented during retrieval. Image sets used during encoding and retrieval sessions were counterbalanced across participants.

The encoding session took place in a single 15-min block. Participants were instructed to indicate whether or not the image contained a building. They were not informed that their memory for the images would be tested later. The images were presented for 2500 ms each, followed by a response screen showing ‘(1) building, (2) no building’. After having made a response with the corresponding keypress, an inter-trial interval of 1000 ms followed before the start of a new trial. The order of presentation for the images was randomized.

The retrieval session tested the participants’ recognition of 160 ‘old’ images presented during encoding randomly intermixed with 80 ‘new’ foil images. A five-point confidence scale was presented below the images: ‘(1) definitely did not see, (2) probably did not see, (3) unsure, (4) probably saw, (5) definitely saw’. Participants were instructed to indicate whether or not they remembered seeing the image from the previous session. Trials ended once a response was made or after a time limit of 5000 ms, then followed by an inter-trial interval of 1000 ms.

Responses were recorded for the encoding and retrieval sessions. Trials with incorrect responses during encoding were excluded from retrieval analyses as they indicate that images were not adequately attended to. Responses from the retrieval session were categorized into four outcome measures: (1) ‘hits’ included confidence ratings of 4 (probably saw) and 5 (definitely saw) to old images, (2) ‘false alarms’ included confidence ratings of 4 and 5 to new images, (3) ‘misses’ included ratings of 1 (definitely did not see) and 2 (probably did not see) to old images, and (4) ‘correct rejections’ included ratings of 1 and 2 to new images. To account for participants’ response bias toward old/new responses, the non-parametric signal detection measure A’ was calculated (Supplementary Eq. (S1)) where 0.5 indicates chance performance 52 .

Factual knowledge task—pretest

The pretest was administered before learning began to gauge prior knowledge of the species to-be-learned. This involved five stages: (1) picture identification: participants identified the name of each species (two options) from an image. (2) General knowledge: 20 two-alternative forced choice questions about general characteristics of amphibians. (3) Specific knowledge: 20 two-alternative forced choice questions on to-be-learned material, similar to questions that would be administered in the final retrieval test. (4) Subjective disgust: participants rated the amount of disgust they felt toward each species on a scale of 1–9 (no disgust to extreme disgust), to control for the influence of emotion on memory. (5) Subjective knowledge: participants rated the amount of prior knowledge they had for each species on a scale of 1–9 (no knowledge to extensive knowledge). All questions were self-paced and presented in a random order.

Factual knowledge task—encoding

Participants learned factual information about 12 species of amphibians consisting of different animal types: three frogs (Poison Dart Frog, Flying Frog, Gray Tree Frog), three toads (Burrowing Toad, Yellow-bellied Toad, Cane Toad), three newts (Alpine Newt, Orange-bellied Newt, Great Crested Newt), and three salamanders (Giant Salamander, Green Salamander, Mud Puppy). Information regarding the characteristics of the species were adapted from their actual biology and behaviours. At the start of the session, participants were informed that all of the information they learned would be tested at a later date with example test questions shown using different species. Additionally, participants were instructed not to discuss or look up information about amphibians outside of the learning sessions.

Learning took place over 3 days. Each day participants underwent two learning blocks in the morning and two blocks in the afternoon, with each block focusing on a different type of animal (e.g., newts). Learning blocks lasted 30 min each. Participants always learned the same pairs of animal in the morning (e.g., newts and frogs) and afternoon (e.g., toads and salamanders) with the order of learning these pairs switched each day. Animal type was counterbalanced across participants for the morning and afternoon sessions. This ensured that information learned in the morning and afternoon was comparable. The repetition of learning over 3 consecutive days was intended to examine the interaction between memory and sleep in an ecologically valid way. It mimicked the way students learn information on separate occasions spaced in time, while also accounting for the effect of repeated cycles of sleep on learning. A single nap or nocturnal sleep episode can inform about how sleep impacts on memory encoding or consolidation, but it is necessary to examine several sleep episodes and learning opportunities together in order to understand the overall impact of daily sleep patterns on long-term memory.

The learning materials consisted of approximately 80 slides of factual information for each animal type presented in the form of numbered points and images. Participants were able to go through the slides at their own pace (moving forwards and backwards freely), but they were advised to keep to a certain pace to ensure that all slides were seen. To facilitate this, a timer was visible throughout the learning session with slides having markers indicating how much time should have passed in 5-min intervals.

To assist with learning, some slides asked participants to write down on a piece of paper what they could recall about the information learned in the previous slides. Participants were also encouraged to take notes which were returned to the experimenter at the end of each block. The final slide of each block instructed participants to use the remaining time to revise the information learned.

At the end of each block participants completed the Karolinska Sleepiness Scale (KSS) and were asked to rate three questions on a 7-point scale: “Was your attention focused on the task or something unrelated to the task? (1 = completely on task, 7 = completely off task)”, “How motivated were you to learn the information? (1 = completely motivated, 7 = completely unmotivated)”, and “How well do you feel you could learn the information? (1 = extremely well, 7 = extremely poorly)”. These scales measured subjective ‘focus’, ‘motivation’, and ‘ability’ respectively. Scores were subsequently inverted for analysis so that higher values represented higher levels for each measure (Supplementary Fig. S1, Table S1).

Factual knowledge task—retrieval

Questions were presented in a two-alternative forced choice format followed by a confidence rating (certain, somewhat certain, guess). The foil was usually the answer to the same question for a different species. Analysis focussed on participants most confident responses (those rated as “certain”) because these were less likely to be contaminated with noise introduced by guessing 2,10,15 . These were corrected for response bias by subtracting incorrect from correct responses. Questions and confidence ratings remained on screen until participants gave a response or after 10 s had elapsed. In total there were 360 questions (90 questions for each animal type). Half of the questions were related to the material learned in the morning and the other half in the afternoon. Questions were presented randomly in six blocks separated by 30 s breaks. Participants were instructed to think carefully about their responses within the given time limit.

Actigraphy

Participants wore an Actiwatch AW-2 (Phillips Respironics Inc., Pittsburgh, PA) during three different periods: (1) 1 week of screening, (2) 1 week immediately before the camp, and (3) two weeks during the camp. Sleep diaries were used to clean and verify the data. Data were scored with Actiware software (version 6.0.2) at 30-s resolution and medium sensitivity.

Polysomnography

Electroencephalography (EEG) was performed via a SOMNOtouch recorder (SOMNOmedics GmbH, Randersacker, Germany) on two channels (C3 and C4) in the international 10–20 system), referenced to contralateral mastoids. Cz and Fpz were used as common reference and ground electrodes respectively. EEG electrode impedances were kept < 5 kΩ and electrooculography (EOG) and submental electromyography (EMG) impedances were kept < 10 kΩ. Pulse oximetry was measured on the first night (B1) to assess for undiagnosed sleep apnoea.

EEG signal was sampled at 256 Hz and band-pass filtered between 0.2 and 35 Hz for EEG, and between 0.2 and 10 Hz for EOG. Automated scoring of sleep stages and artefacts were performed using Z3Score 53 (https://z3score.com) in conjunction with the FASST EEG toolbox, and was also visually checked by trained staff following criteria from the American Academy of Sleep Medicine Manual for the Scoring of Sleep and Associated Events 23 .

For each recording, the following parameters were computed: TST, N2 latency (time from lights off to N2 onset), durations of N1, N2, N3 and REM sleep, as well as sleep efficiency and WASO. As a measure of homeostatic sleep pressure, we computed slow wave energy (SWE 0.6–4 Hz) in the first hour of sleep from N2 onset by integrating power in the delta band summed across all NREM epochs, and normalized to SWE in the first hour of B2 41 . Automatic sleep spindle detection analysis was performed using the Wonambi Python package, v5.24 (https://wonambi-python.github.io) with an automated algorithm 54 . Spindle count was computed for NREM epochs using C3-A2 electrodes.

Statistical analysis

For picture encoding we assessed accuracy during encoding and memory at retrieval (A’) via a 2 × 2 ANOVA with between-subject factors of sleep schedule (split/continuous) and duration (6.5 h/8 h).

For the factual knowledge task, we assessed prior knowledge of the stimuli via one-way ANOVA on test scores for general knowledge, specific knowledge, picture identification, as well as subjective knowledge and disgust. Certain memory (correct-incorrect) during retrieval was analysed with a 2 × 2 × 2 mixed ANOVA including schedule (split/continuous), duration (6.5 h/8 h), and the within-subject factor of time (morning/afternoon). Paired and independent sample t-tests were used for follow-up comparisons. Subjective measures (alertness, focus, ability, motivation) were assessed via the same 2 × 2 × 2 mixed ANOVA.

For PSG, total sleep time and duration spent in sleep stages across 24 h were compared using a one-way ANOVA, and independent t-tests. Pearson’s correlations were performed between memory and sleep measures: picture encoding used the total sleep period 24 h prior to encoding (M15) and recovery sleep shortly after encoding (R11). Factual knowledge learning took place from M21 to M23, therefore we averaged the PSG-assessed sleep parameters obtained across M21 and M23.

All effect sizes were computed with Cohen’s d (d) for t-tests of key comparisons. All statistical tests were two-tailed with significance level of p < 0.05 (uncorrected).


Sleep Deprivation, Psychosis and Mental Efficiency

Today, average young adults report sleeping about seven to seven and one-half hours each night. Compare this to sleep patterns in 1910, before the electric lightbulb, the average person slept nine hours each night. This means that today's population sleeps one to two hours less than people did early in the century.

Today, average young adults report sleeping about seven to seven and one-half hours each night. Compare this to sleep patterns in 1910, before the electric lightbulb, the average person slept nine hours each night. This means that today's population sleeps one to two hours less than people did early in the century (Webb and Agnew, 1975).

Because of the advent of the lightbulb, people sleep 500 hours less each year than they used to. Unfortunately, our current "sleep diet" is significantly less than evolution intended. Most other primates (e.g., apes and monkeys) have a 24-hour sleep and activity cycle that is similar to that of humans who live in cultures where the siesta is still practiced. These animals have a long sleep at night, and a shorter sleep in the midafternoon, with a daily sleep total of about 10 hours. Humans seem to naturally need about the same amount of sleep. For instance, when the pressure of work, alarm clocks, social schedules and advanced technology is removed, people tend to sleep longer. Thus, in many less industrialized societies, the total daily sleep time is still around nine to 10 hours as it is for people when they are on unstructured holidays (Coren, 1996a).

Confirmation of these natural sleep durations comes from Palinkas, Suedfeld and Steel (1995). These researchers spent a summer above the arctic circle where there is continuous light 24 hours a day. All watches, clocks and other timekeeping devices were removed, and only the station's computers tracked the times that the team went to sleep and awakened. Individual researchers did their work, and chose when to sleep or wake according to their "body time." At the end of the experiment, they found that their overall average sleep daily time was 10.3 hours. Every member of the team showed an increase in sleep time, with the shortest logging in at 8.8 hours a day, and the longest almost 12 hours a day. This study, like many others, seems to suggest that our biological need for sleep might be closer to the 10 hours per day that is typical of monkeys and apes living in the wild, than the 7 to 7.5 hours typical of humans in today's high-tech, clock-driven lifestyle.

Psychological researchers have tended to minimize the effects of sleep insufficiency, acknowledging that society may be getting too little sleep, but treating the effects of this sleep deprivation as nothing more significant than an inconvenience which makes people feel a bit tired now and then.

This view is incorrect. Recent research suggests that each day with insufficient sleep increases our sleep debt and, when this sleep debt becomes large enough, noticeable problems appear (Coren, 1996a). These sleep-debt-related problems are most predictable at certain times of the day. This is because the efficiency of our physical and mental functions show cyclic increases and decreases in the form of circadian rhythms. While our major sleep/wakefulness rhythm has a cycle length of roughly 24 hours, there are shorter cycles as well, with the most important of these being a secondary sleep/wakefulness cycle that is around 12 hours.

Because of these cycles, the pressure to fall asleep is greatest in the morning, between 1 and 4 a.m. In addition there is a less pronounced, but still noticeable, increase in sleepiness 12 hours later, between 1 and 4 p.m. It is this afternoon low point that makes you feel sleepy after lunch, not the meal that you may have just eaten. It probably also was the original reason for the afternoon nap or siesta.

People who are operating with a sleep debt are less efficient, and this inefficiency is most noticeable when the circadian cycle is at its lowest ebb. Among the common consequences of a large sleep debt are attentional lapses, reduced short-term memory capacity, impaired judgment and the occurrence of "microsleeps."

A microsleep is a short period of time, usually between 10 seconds to a minute in length, in which the brain actually enters a sleep state, regardless of what the person is doing at the time. The affected individual often does not know that this momentary blackout has occurred. The effects of these microsleeps combined with attentional lapses, however, can be dramatic.

There is now evidence that many major disasters have been due to sleep-debt related effects. The evidence shows that these include the oil spill of the Exxon Valdez, the nuclear accidents at Chernobyl and Three Mile Island, and the loss of the space shuttle Challenger (Coren, 1996a).

In fact, our societal sleep debt is so great that simply losing one additional hour of sleep due to the spring shift to daylight savings time can increase traffic accident rates by 7% (Coren, 1996b) and death rates due to all accidents by 6.5% (Coren, 1996c).

When sleep deprivation becomes great enough, the effects mimic those of psychosis. The failure of the scientific world to recognize this is due to some extent to the folklore that has grown up around the sleepless marathon of high school student Randy Gardner in 1964.

To gain an entry into the Guiness Book of World Records, he remained awake for 264 hours (11 days). Summaries of this case usually report that Gardner suffered no hallucinations, no paranoia or other negative mood changes, and that his mental, motor and sensory abilities were quite good throughout the entire episode. This conclusion is so widespread that it has now become a stock "fact" presented in virtually any psychology or psychiatry book that has a chapter on sleep.

This conclusion seems to be based on two items of information. The first was the observation that there were no obvious lasting physical or mental problems encountered by Gardner when he began to sleep again. The second was based upon observations of researcher William Dement (Dement, 1992), who interviewed Gardner on Day 10 of the experiment. He reported that he took Gardner to a restaurant and then played pinball with him, noting that Gardner played the game well and even won. Lt. Cmdr. John J. Ross of the U.S. Navy Medical Neuropsychiatric Research Unit in San Diego, who was called in by Gardner's worried parents to monitor his condition, tells a quite different story (Ross, 1965). Gardner's symptoms that Ross reported included:

  • Day 2: Difficulty focusing eyes and signs of astereognosis (difficulty recognizing objects only by touch).
  • Day 3: Moodiness, some signs of ataxia (inability to repeat simple tongue twisters).
  • Day 4: Irritability and uncooperative attitude, memory lapses and difficulty concentrating. Gardner's first hallucination was that a street sign was a person, followed by a delusional episode in which he imagined that he was a famous black football player.
  • Day 5: More hallucinations (e.g., seeing a path extending from the room in front of him down through a quiet forest). These were sometimes described as "hypnagogic reveries" since Gardner recognized, at least after a short while, that the visions were illusionary in nature.
  • Day 6: Speech slowing and difficulty naming common objects.
  • Day 7 and 8: Irritability, speech slurring and increased memory lapses.
  • Day 9: Episodes of fragmented thinking frequently beginning, but not finishing, his sentences.
  • Day 10: Paranoia focused on a radio show host who Gardner felt was trying to make him appear foolish because he ws having difficulty remembering some details about his vigil.
  • Day 11: Expressionless appearance, speech slurred and without intonation had to be encouraged to talk to get him to respond at all. His attention span was very short and his mental abilities were diminished. In a serial sevens test, where the respondent starts with the number 100 and proceeds downward by subtracting seven each time, Gardner got back to 65 (only five subtractions) and then stopped. When asked why he had stopped he claimed that he couldn't remember what he was supposed to be doing.

In many respects Gardner's symptoms were similar to those experienced by a New York disk jockey, Peter Tripp, who endured a 200-hour sleepless marathon to raise money for the March of Dimes. During the course of his ordeal his thoughts became increasingly distorted and there were marked periods of irrationality. By the end of four days he could not successfully execute simple tests requiring focused attention. In addition, he began to have hallucinations and distorted visual perceptions. At one point Tripp became quite upset when he thought that the spots on a table were insects. He thought that there were spiders crawling around the booth and even once complained that they had spun cobwebs on his shoes.

He showed the same increasing moodiness and paranoia that Gardner did. On his last day, a neurologist was called to examine Tripp before sending him home. When Tripp looked up at this doctor in his dark, old-fashioned suit, he had the delusion that the doctor was really an undertaker who was about to bury him alive. Overtaken with fear, he let loose a scream and bolted for the door. Half-dressed, Tripp ran down the hall with doctors and psychologists in pursuit. He could no longer distinguish the difference between reality and nightmare.

This same pattern of mental deterioration that mimicks psychotic symptoms appears in several more systematic studies of sleep deprivation and extreme sleep debt. Thus, prolonged sleep deprivation does lead to the appearance of serious mental symptoms. In addition, even moderate amounts of sleep deprivation can lead to losses in mental efficiency that can threaten public and personal safety.

(According to a 1995 report by the U.S. Department of Transportation, 100,000 sleep-related traffic accidents claim some 1,500 American lives each year. The National Commission on Sleep Disorders Research has reported that sleep-related accidents, and sleep disorders which impact work productivity, cost the American economy between $100 and $150 billion each year-Ed.)

References:

References

Coren S (1996a), Sleep Thieves. New York: Free Press.

Coren S (1996b), Daylight savings time and traffic accidents. New Eng J Med 334:924.

Coren S (1996c), Accidental death and the shift to daylight savings time. Percept Mot Skills 83:921-922.

Dement WC (1992), The Sleepwatchers. Stanford, Calif.: Stanford Alumni Association.

Palinkas LA, Suedfeld P, Steel GD (1995), Psychological functioning among members of a small polar expedition. Avia Space Environ Med 66:943-950.

Ross JJ (1965), Neurological findings after prolonged sleep deprivation. Arch Neurol 12:399-403.
Webb WB, Agnew HW (1975), Are we chronically sleep deprived? Bull the Psychonomic Soc 6:47-48.


Music listening near bedtime disruptive to sleep

Most people listen to music throughout their day and often near bedtime to wind down. But can that actually cause your sleep to suffer? When sleep researcher Michael Scullin, Ph.D., associate professor of psychology and neuroscience at Baylor University, realized he was waking in the middle of the night with a song stuck in his head, he saw an opportunity to study how music -- and particularly stuck songs -- might affect sleep patterns.

Scullin's recent study, published in Psychological Science, investigated the relationship between music listening and sleep, focusing on a rarely-explored mechanism: involuntary musical imagery, or "earworms," when a song or tune replays over and over in a person's mind. These commonly happen while awake, but Scullin found that they also can happen while trying to sleep.

"Our brains continue to process music even when none is playing, including apparently while we are asleep," Scullin said. "Everyone knows that music listening feels good. Adolescents and young adults routinely listen to music near bedtime. But sometimes you can have too much of a good thing. The more you listen to music, the more likely you are to catch an earworm that won't go away at bedtime. When that happens, chances are your sleep is going to suffer."

People who experience earworms regularly at night -- one or more times per week -- are six times as likely to have poor sleep quality compared to people who rarely experience earworms. Surprisingly, the study found that some instrumental music is more likely to lead to earworms and disrupt sleep quality than lyrical music.

The study involved a survey and a laboratory experiment. The survey involved 209 participants who completed a series of surveys on sleep quality, music listening habits and earworm frequency, including how often they experienced an earworm while trying to fall asleep, waking up in the middle of the night and immediately upon waking in the morning.

In the experimental study, 50 participants were brought into Scullin's Sleep Neuroscience and Cognition Laboratory at Baylor, where the research team attempted to induce earworms to determine how it affected sleep quality. Polysomnography -- a comprehensive test and the gold standard measurement for sleep -- was used to record the participants' brain waves, heart rate, breathing and more while they slept.

"Before bedtime, we played three popular and catchy songs -- Taylor Swift's 'Shake It Off,' Carly Rae Jepsen's 'Call Me Maybe' and Journey's 'Don't Stop Believin'," Scullin said. "We randomly assigned participants to listen to the original versions of those songs or the de-lyricized instrumental versions of the songs. Participants responded whether and when they experienced an earworm. Then we analyzed whether that impacted their nighttime sleep physiology. People who caught an earworm had greater difficulty falling asleep, more nighttime awakenings, and spent more time in light stages of sleep."

Additionally, EEG readings -- records of electrical activity in the brain -- from the experimental study were quantitatively analyzed to examine physiological markers of sleep-dependent memory consolidation. Memory consolidation is the process by which temporary memories are spontaneously reactived during sleep and transformed into a more long-term form.

"We thought that people would have earworms at bedtime when they were trying to fall asleep, but we certainly didn't know that people would report regularly waking up from sleep with an earworm. But we saw that in both the survey and experimental study," he said.

Participants who had a sleep earworm showed more slow oscillations during sleep, a marker of memory reactivation. The increase in slow oscillations was dominant over the region corresponding to the primary auditory cortex which is implicated in earworm processing when people are awake.

"Almost everyone thought music improves their sleep, but we found those who listened to more music slept worse," Scullin said. "What was really surprising was that instrumental music led to worse sleep quality -- instrumental music leads to about twice as many earworms."

The study found that individuals with greater music listening habits experienced persistent earworms and a decline in sleep quality. These results are contrary to the idea of music as a hypnotic that might help sleep. Health organizations commonly recommend listening to quiet music before bedtime -- recommendations that largely arise from self-reported studies. Instead, Scullin has objectively measured that the sleeping brain continues to process music for several hours, even after the music stops.

Knowing that earworms negatively affect sleep, Scullin recommends first trying to moderate music listening or taking occasional breaks if bothered by earworms. Timing of music also is important -- try to avoid it before bed.

"If you commonly pair listening to music while being in bed, then you'll have that association where being in that context might trigger an earworm even when you're not listening to music, such as when you're trying to fall asleep," he said.

Another way to get rid of an earworm is to engage in cognitive activity -- fully focusing on a task, problem or activity helps to distract your brain from earworms. Near bedtime, rather than engaging in a demanding activity or something that would disrupt your sleep, like watching TV or playing video games, Scullin suggests spending five to 10 minutes writing out a to-do list and putting thoughts to paper. A previous study by Scullin -- partially funded by a National Institutes of Health grant and the Sleep Research Society Foundation -- found that participants who took five minutes to write down upcoming tasks before bed helped "offload" those worrying thoughts about the future and led to faster sleep.


Sleeping in Short Periods Disrupts Natural Rhythms

If you suffer from inadequate rest, either of insufficient quantity or of poor quality, you are likely to experience excessive daytime sleepiness. This drowsiness can make you capable of falling asleep at almost any time. Rather than sleeping in one consolidated period of sleep overnight, you may sleep in short periods. This affects natural circadian rhythms and disrupts normal sleep cycles.

Our desire for sleep increases the longer we are awake. This is called our homeostatic sleep drive. This gradually accumulating desire for sleep builds the longer that we stay awake. We are able to resist this for many hours (even days), but eventually, the desire for sleep overwhelms us and we fall asleep. This may be due to an accumulation of neurotransmitters, chemicals in the brain that function as signals between nerve cells.

The second element that contributes to our desire for sleep is the circadian rhythm. As creatures who are typically awake during the day and asleep at night, the circadian rhythm reinforces this sleep pattern. In nocturnal animals, such as rats, the reverse pattern is seen. Various hormones in the body follow a circadian pattern. Melatonin, for example, peaks overnight. Another hormone, cortisol, plays an important role in waking us up in the morning.

These two processes come together to encourage increased drowsiness and a strong desire for sleep overnight. However, our behaviors may disrupt these natural tendencies.


Why sleep persists is fairly easy, why it is needed is an unknown.

Sleep appears to be necessary in any organism with a brain, that is anything with any kind of concentration of neurons. That is when denied it said organisms die. So all that has to happen is that the benefits of a brain outweighs the cost of sleep.

The length of sleep needed correlates with brain size, at least the REM part of sleep other parts correlate with metabolic rates. Now this can be seemingly confounded in larger complex brains (birds and mammals especially) when organisms start folding the brain to increase neuron density without increasing overall size. In this case these organisms are increasing hte "size" of the brain without making the brain larger, by increasing density. Worse some organisms "sleep" for long periods, but only a short portion of that time involves the neural activity associated with sleep (like REM), sleep in more complex animals (those with very large complex brains) contains many functions.

In organisms with tiny brains and slow metabolisms (aka the earliest things with brains) sleep does not take very long so the cost is minimal, the benefits of a brain (and thus learning) can be high. Later as brains get larger the cost goes up but so does the benefit, if it was possible for a brain to be retained without the need for sleep it should have been selected for at this stage. So in all likelihood the need for sleep is something fundamental to how neurons function and can't be changed without seriously disruptioin their function. It is not uncommon for unfavorable things to get locked in evolutionarily in this way, the cost to change them (in this case the loss of brain function) is far larger than the cost of sleep so selection keeps it around.

Now the complexity of sleep makes sense, if you already have this required period of downtime, it makes sense evolutionarily to tack on anything else that would be best done during said time. Better to use the triggers and time for existing downtime activities for any others that get added then have even more downtime. So now we have a slew of confounding factors that muddy studies and make it hard to tell what parts are essential.

We don't know why sleep is necessary, there are many ideas but not a lot of evidence. Given the complexity of sleep this is not surprising, teasing out which functions are fundamental is not easy. There is a current leaning towards it being necessary to clear out metabolites that disrupt neural function, it appears this process is highly disruptive to the brain if the brain is awake, to the point simply shutting down brain activity (movement in particular) is far safer for the organism. But as with all research this is very preliminary and sleep is poorly understood so acceptance is and should be very tentative.


Contents

The most pronounced physiological changes in sleep occur in the brain. [8] The brain uses significantly less energy during sleep than it does when awake, especially during non-REM sleep. In areas with reduced activity, the brain restores its supply of adenosine triphosphate (ATP), the molecule used for short-term storage and transport of energy. [9] In quiet waking, the brain is responsible for 20% of the body's energy use, thus this reduction has a noticeable effect on overall energy consumption. [10]

Sleep increases the sensory threshold. In other words, sleeping persons perceive fewer stimuli, but can generally still respond to loud noises and other salient sensory events. [10] [8]

During slow-wave sleep, humans secrete bursts of growth hormone. All sleep, even during the day, is associated with secretion of prolactin. [11]

Key physiological methods for monitoring and measuring changes during sleep include electroencephalography (EEG) of brain waves, electrooculography (EOG) of eye movements, and electromyography (EMG) of skeletal muscle activity. Simultaneous collection of these measurements is called polysomnography, and can be performed in a specialized sleep laboratory. [12] [13] Sleep researchers also use simplified electrocardiography (EKG) for cardiac activity and actigraphy for motor movements. [13]

Non-REM and REM sleep

Sleep is divided into two broad types: non-rapid eye movement (non-REM or NREM) sleep and rapid eye movement (REM) sleep. Non-REM and REM sleep are so different that physiologists identify them as distinct behavioral states. Non-REM sleep occurs first and after a transitional period is called slow-wave sleep or deep sleep. During this phase, body temperature and heart rate fall, and the brain uses less energy. [8] REM sleep, also known as paradoxical sleep, represents a smaller portion of total sleep time. It is the main occasion for dreams (or nightmares), and is associated with desynchronized and fast brain waves, eye movements, loss of muscle tone, [2] and suspension of homeostasis. [14]

The sleep cycle of alternate NREM and REM sleep takes an average of 90 minutes, occurring 4–6 times in a good night's sleep. [13] [15] The American Academy of Sleep Medicine (AASM) divides NREM into three stages: N1, N2, and N3, the last of which is also called delta sleep or slow-wave sleep. [16] The whole period normally proceeds in the order: N1 → N2 → N3 → N2 → REM. REM sleep occurs as a person returns to stage 2 or 1 from a deep sleep. [2] There is a greater amount of deep sleep (stage N3) earlier in the night, while the proportion of REM sleep increases in the two cycles just before natural awakening. [13]

Awakening

Awakening can mean the end of sleep, or simply a moment to survey the environment and readjust body position before falling back asleep. Sleepers typically awaken soon after the end of a REM phase or sometimes in the middle of REM. Internal circadian indicators, along with a successful reduction of homeostatic sleep need, typically bring about awakening and the end of the sleep cycle. [17] Awakening involves heightened electrical activation in the brain, beginning with the thalamus and spreading throughout the cortex. [17]

During a night's sleep, a small amount of time is usually spent in a waking state. As measured by electroencephalography, young females are awake for 0–1% of the larger sleeping period young males are awake for 0–2%. In adults, wakefulness increases, especially in later cycles. One study found 3% awake time in the first ninety-minute sleep cycle, 8% in the second, 10% in the third, 12% in the fourth, and 13–14% in the fifth. Most of this awake time occurred shortly after REM sleep. [17]

Today, many humans wake up with an alarm clock [18] however, people can also reliably wake themselves up at a specific time with no need for an alarm. [17] Many sleep quite differently on workdays versus days off, a pattern which can lead to chronic circadian desynchronization. [19] [18] Many people regularly look at television and other screens before going to bed, a factor which may exacerbate disruption of the circadian cycle. [20] [21] Scientific studies on sleep have shown that sleep stage at awakening is an important factor in amplifying sleep inertia. [22]

Sleep timing is controlled by the circadian clock (Process C), sleep-wake homeostasis (Process S), and to some extent by the individual will.

Circadian clock

Sleep timing depends greatly on hormonal signals from the circadian clock, or Process C, a complex neurochemical system which uses signals from an organism's environment to recreate an internal day–night rhythm. Process C counteracts the homeostatic drive for sleep during the day (in diurnal animals) and augments it at night. [23] [19] The suprachiasmatic nucleus (SCN), a brain area directly above the optic chiasm, is presently considered the most important nexus for this process however, secondary clock systems have been found throughout the body.

An organism whose circadian clock exhibits a regular rhythm corresponding to outside signals is said to be entrained an entrained rhythm persists even if the outside signals suddenly disappear. If an entrained human is isolated in a bunker with constant light or darkness, he or she will continue to experience rhythmic increases and decreases of body temperature and melatonin, on a period that slightly exceeds 24 hours. Scientists refer to such conditions as free-running of the circadian rhythm. Under natural conditions, light signals regularly adjust this period downward, so that it corresponds better with the exact 24 hours of an Earth day. [18] [24] [25]

The circadian clock exerts constant influence on the body, affecting sinusoidal oscillation of body temperature between roughly 36.2 °C and 37.2 °C. [25] [26] The suprachiasmatic nucleus itself shows conspicuous oscillation activity, which intensifies during subjective day (i.e., the part of the rhythm corresponding with daytime, whether accurately or not) and drops to almost nothing during subjective night. [27] The circadian pacemaker in the suprachiasmatic nucleus has a direct neural connection to the pineal gland, which releases the hormone melatonin at night. [27] Cortisol levels typically rise throughout the night, peak in the awakening hours, and diminish during the day. [11] [28] Circadian prolactin secretion begins in the late afternoon, especially in women, and is subsequently augmented by sleep-induced secretion, to peak in the middle of the night. Circadian rhythm exerts some influence on the nighttime secretion of growth hormone. [11]

The circadian rhythm influences the ideal timing of a restorative sleep episode. [18] [29] Sleepiness increases during the night. REM sleep occurs more during body temperature minimum within the circadian cycle, whereas slow-wave sleep can occur more independently of circadian time. [25]

The internal circadian clock is profoundly influenced by changes in light, since these are its main clues about what time it is. Exposure to even small amounts of light during the night can suppress melatonin secretion, and increase body temperature and wakefulness. Short pulses of light, at the right moment in the circadian cycle, can significantly 'reset' the internal clock. [26] Blue light, in particular, exerts the strongest effect, [19] leading to concerns that electronic media use before bed may interfere with sleep. [20]

Modern humans often find themselves desynchronized from their internal circadian clock, due to the requirements of work (especially night shifts), long-distance travel, and the influence of universal indoor lighting. [25] Even if they have sleep debt, or feel sleepy, people can have difficulty staying asleep at the peak of their circadian cycle. Conversely, they can have difficulty waking up in the trough of the cycle. [17] A healthy young adult entrained to the sun will (during most of the year) fall asleep a few hours after sunset, experience body temperature minimum at 6 a.m., and wake up a few hours after sunrise. [25]

Process S

Generally speaking, the longer an organism is awake, the more it feels a need to sleep ("sleep debt"). This driver of sleep is referred to as Process S. The balance between sleeping and waking is regulated by a process called homeostasis. Induced or perceived lack of sleep is called sleep deprivation.

Process S is driven by the depletion of glycogen and accumulation of adenosine in the forebrain that disinhibits the ventrolateral preoptic nucleus, allowing for inhibition of the ascending reticular activating system. [30]

Sleep deprivation tends to cause slower brain waves in the frontal cortex, shortened attention span, higher anxiety, impaired memory, and a grouchy mood. Conversely, a well-rested organism tends to have improved memory and mood. [31] Neurophysiological and functional imaging studies have demonstrated that frontal regions of the brain are particularly responsive to homeostatic sleep pressure. [32]

There is disagreement on how much sleep debt is possible to accumulate, and whether sleep debt is accumulated against an individual's average sleep or some other benchmark. It is also unclear whether the prevalence of sleep debt among adults has changed appreciably in the industrialized world in recent decades. Sleep debt does show some evidence of being cumulative. Subjectively, however, humans seem to reach maximum sleepiness after 30 hours of waking up. [25] It is likely that in Western societies, children are sleeping less than they previously have. [33]

One neurochemical indicator of sleep debt is adenosine, a neurotransmitter that inhibits many of the bodily processes associated with wakefulness. Adenosine levels increase in the cortex and basal forebrain during prolonged wakefulness, and decrease during the sleep-recovery period, potentially acting as a homeostatic regulator of sleep. [34] [35] Coffee and caffeine temporarily block the effect of adenosine, prolong sleep latency, and reduce total sleep time and quality. [36]

Social timing

Humans are also influenced by aspects of social time, such as the hours when other people are awake, the hours when work is required, the time on the clock, etc. Time zones, standard times used to unify the timing for people in the same area, correspond only approximately to the natural rising and setting of the sun. The approximate nature of the time zone can be shown with China, a country which used to span five time zones and now officially uses only one (UTC+8). [18]

Distribution

In polyphasic sleep, an organism sleeps several times in a 24-hour cycle, whereas in monophasic sleep this occurs all at once. Under experimental conditions, humans tend to alternate more frequently between sleep and wakefulness (i.e., exhibit more polyphasic sleep) if they have nothing better to do. [25] Given a 14-hour period of darkness in experimental conditions, humans tended towards bimodal sleep, with two sleep periods concentrated at the beginning and at the end of the dark time. Bimodal sleep in humans was more common before the industrial revolution. [28]

Different characteristic sleep patterns, such as the familiarly so-called "early bird" and "night owl", are called chronotypes. Genetics and sex have some influence on chronotype, but so do habits. Chronotype is also liable to change over the course of a person's lifetime. Seven-year-olds are better disposed to wake up early in the morning than are fifteen-year-olds. [19] [18] Chronotypes far outside the normal range are called circadian rhythm sleep disorders. [37]

The siesta habit has recently been associated with a 37% lower coronary mortality, possibly due to reduced cardiovascular stress mediated by daytime sleep. [38] Short naps at mid-day and mild evening exercise were found to be effective for improved sleep, cognitive tasks, and mental health in elderly people. [39]

Genetics

Monozygotic (identical) but not dizygotic (fraternal) twins tend to have similar sleep habits. Neurotransmitters, molecules whose production can be traced to specific genes, are one genetic influence on sleep that can be analyzed. The circadian clock has its own set of genes. [40] Genes which may influence sleep include ABCC9, DEC2, Dopamine receptor D2 [41] and variants near PAX 8 and VRK2. [42]

Quality

The quality of sleep may be evaluated from an objective and a subjective point of view. Objective sleep quality refers to how difficult it is for a person to fall asleep and remain in a sleeping state, and how many times they wake up during a single night. Poor sleep quality disrupts the cycle of transition between the different stages of sleep. [43] Subjective sleep quality in turn refers to a sense of being rested and regenerated after awaking from sleep. A study by A. Harvey et al. (2002) found that insomniacs were more demanding in their evaluations of sleep quality than individuals who had no sleep problems. [44]

Homeostatic sleep propensity (the need for sleep as a function of the amount of time elapsed since the last adequate sleep episode) must be balanced against the circadian element for satisfactory sleep. [45] [46] Along with corresponding messages from the circadian clock, this tells the body it needs to sleep. [47] The timing is correct when the following two circadian markers occur after the middle of the sleep episode and before awakening: [48] maximum concentration of the hormone melatonin, and minimum core body temperature.

Human sleep-needs vary by age and amongst individuals sleep is considered to be adequate when there is no daytime sleepiness or dysfunction. Moreover, self-reported sleep duration is only moderately correlated with actual sleep time as measured by actigraphy, [50] and those affected with sleep state misperception may typically report having slept only four hours despite having slept a full eight hours. [51]

Researchers have found that sleeping 6–7 hours each night correlates with longevity and cardiac health in humans, though many underlying factors may be involved in the causality behind this relationship. [52] [53] [54] [55] [42] [56] [57]

Sleep difficulties are furthermore associated with psychiatric disorders such as depression, alcoholism, and bipolar disorder. [58] Up to 90 percent of adults with depression are found to have sleep difficulties. Dysregulation detected by EEG includes disturbances in sleep continuity, decreased delta sleep and altered REM patterns with regard to latency, distribution across the night and density of eye movements. [59]

Sleep duration can also vary according to season. Up to 90% of people report longer sleep duration in winter, which may lead to more pronounced seasonal affective disorder. [60] [61]

Children

By the time infants reach the age of two, their brain size has reached 90 percent of an adult-sized brain [62] a majority of this brain growth has occurred during the period of life with the highest rate of sleep. The hours that children spend asleep influence their ability to perform on cognitive tasks. [63] [64] Children who sleep through the night and have few night waking episodes have higher cognitive attainments and easier temperaments than other children. [64] [65] [66]

Sleep also influences language development. To test this, researchers taught infants a faux language and observed their recollection of the rules for that language. [67] Infants who slept within four hours of learning the language could remember the language rules better, while infants who stayed awake longer did not recall those rules as well. There is also a relationship between infants' vocabulary and sleeping: infants who sleep longer at night at 12 months have better vocabularies at 26 months. [66]

Recommendations

Children need many hours of sleep per day in order to develop and function properly: up to 18 hours for newborn babies, with a declining rate as a child ages. [47] Early in 2015, after a two-year study, [68] the National Sleep Foundation in the US announced newly-revised recommendations as shown in the table below.

Hours of sleep required for each age group [68]
Age and condition Sleep needs
Newborns (0–3 months) 14 to 17 hours
Infants (4–11 months) 12 to 15 hours
Toddlers (1–2 years) 11 to 14 hours
Preschoolers (3–4 years) 10 to 13 hours
School-age children (5–12 years) 9 to 11 hours
Teenagers (13–17 years) 8 to 10 hours
Adults (18–64 years) 7 to 9 hours
Older Adults (65 years and over) 7 to 8 hours

Restoration

The human organism physically restores itself during sleep, occurring mostly during slow-wave sleep during which body temperature, heart rate, and brain oxygen consumption decrease. In both the brain and body, the reduced rate of metabolism enables countervailing restorative processes. [69] The brain requires sleep for restoration, whereas these processes can take place during quiescent waking in the rest of the body. [ citation needed ] The essential function of sleep may be its restorative effect on the brain: "Sleep is of the brain, by the brain and for the brain." [70] This theory is strengthened by the fact that sleep is observed to be a necessary behavior across most of the animal kingdom, including some of the least evolved animals which have no need for other functions of sleep, such as memory consolidation or dreaming. [6]

While awake, brain metabolism generates end products, such as reactive oxygen species, which may be damaging to brain cells and inhibit their proper function. During sleep, metabolic rates decrease and reactive oxygen species generation is reduced, enabling restorative processes. The sleeping brain has been shown to remove metabolic end products at a faster rate than during an awake state. [ citation needed ] The mechanism for this removal appears to be the glymphatic system. [71] Sleep may facilitate the synthesis of molecules that help repair and protect the brain from metabolic end products generated during waking. [72] Anabolic hormones, such as growth hormones, are secreted preferentially during sleep. The brain concentration of glycogen increases during sleep, and is depleted through metabolism during wakefulness. [69]

The effect of sleep duration on somatic growth is not completely known. One study recorded growth, height, and weight, as correlated to parent-reported time in bed in 305 children over a period of nine years (age 1–10). It was found that "the variation of sleep duration among children does not seem to have an effect on growth." [73] Slow-wave sleep affects growth hormone levels in adult men. [11] During eight hours of sleep, men with a high percentage of slow-wave sleep (SWS) (average 24%) also had high growth hormone secretion, while subjects with a low percentage of SWS (average 9%) had low growth hormone secretion. [74]

Memory processing

It has been widely accepted that sleep must support the formation of long-term memory, and generally increasing previous learning and experiences recalls. However, its benefit seems to depend on the phase of sleep and the type of memory. [75] For example, declarative and procedural memory-recall tasks applied over early and late nocturnal sleep, as well as wakefulness controlled conditions, have been shown that declarative memory improves more during early sleep (dominated by SWS) while procedural memory during late sleep (dominated by REM sleep) does so. [76] [77]

With regard to declarative memory, the functional role of SWS has been associated with hippocampal replays of previously encoded neural patterns that seem to facilitate long-term memory consolidation. [76] [77] This assumption is based on the active system consolidation hypothesis, which states that repeated reactivations of newly-encoded information in the hippocampus during slow oscillations in NREM sleep mediate the stabilization and gradual integration of declarative memory with pre-existing knowledge networks on the cortical level. [78] It assumes the hippocampus might hold information only temporarily and in a fast-learning rate, whereas the neocortex is related to long-term storage and a slow-learning rate. [76] [77] [79] [80] [81] This dialogue between the hippocampus and neocortex occurs in parallel with hippocampal sharp-wave ripples and thalamo-cortical spindles, synchrony that drives the formation of the spindle-ripple event which seems to be a prerequisite for the formation of long-term memories. [77] [79] [81] [82]

Reactivation of memory also occurs during wakefulness and its function is associated with serving to update the reactivated memory with newly-encoded information, whereas reactivations during SWS are presented as crucial for memory stabilization. [77] Based on targeted memory reactivation (TMR) experiments that use associated memory cues to triggering memory traces during sleep, several studies have been reassuring the importance of nocturnal reactivations for the formation of persistent memories in neocortical networks, as well as highlighting the possibility of increasing people’s memory performance at declarative recalls. [76] [80] [81] [82] [83]

Furthermore, nocturnal reactivation seems to share the same neural oscillatory patterns as reactivation during wakefulness, processes which might be coordinated by theta activity. [84] During wakefulness, theta oscillations have been often related to successful performance in memory tasks, and cued memory reactivations during sleep have been showing that theta activity is significantly stronger in subsequent recognition of cued stimuli as compared to uncued ones, possibly indicating a strengthening of memory traces and lexical integration by cuing during sleep. [85] However, the beneficial effect of TMR for memory consolidation seems to occur only if the cued memories can be related to prior knowledge. [86]

Dreaming

During sleep, especially REM sleep, humans tend to experience dreams. These are elusive and mostly unpredictable first-person experiences which seem logical and realistic to the dreamer while they are in progress, despite their frequently bizarre, irrational, and/or surreal qualities that become apparent when assessed after waking. Dreams often seamlessly incorporate concepts, situations, people, and objects within a person's mind that would not normally go together. They can include apparent sensations of all types, especially vision and movement. [87]

Dreams tend to rapidly fade from memory after waking. Some people choose to keep a dream journal, which they believe helps them build dream recall and facilitate the ability to experience lucid dreams.

People have proposed many hypotheses about the functions of dreaming. Sigmund Freud postulated that dreams are the symbolic expression of frustrated desires that have been relegated to the unconscious mind, and he used dream interpretation in the form of psychoanalysis in attempting to uncover these desires. [88]

Counterintuitively, penile erections during sleep are not more frequent during sexual dreams than during other dreams. [89] The parasympathetic nervous system experiences increased activity during REM sleep which may cause erection of the penis or clitoris. In males, 80% to 95% of REM sleep is normally accompanied by partial to full penile erection, while only about 12% of men's dreams contain sexual content. [90]

John Allan Hobson and Robert McCarley propose that dreams are caused by the random firing of neurons in the cerebral cortex during the REM period. Neatly, this theory helps explain the irrationality of the mind during REM periods, as, according to this theory, the forebrain then creates a story in an attempt to reconcile and make sense of the nonsensical sensory information presented to it. This would explain the odd nature of many dreams. [91]

Using antidepressants, [ clarification needed ] acetaminophen, ibuprofen, or alcoholic beverages is thought to potentially suppress dreams, whereas melatonin may have the ability to encourage them. [92]

Insomnia

Insomnia is a general term for difficulty falling asleep and/or staying asleep. Insomnia is the most common sleep problem, with many adults reporting occasional insomnia, and 10–15% reporting a chronic condition. [93] Insomnia can have many different causes, including psychological stress, a poor sleep environment, an inconsistent sleep schedule, or excessive mental or physical stimulation in the hours before bedtime. Insomnia is often treated through behavioral changes like keeping a regular sleep schedule, avoiding stimulating or stressful activities before bedtime, and cutting down on stimulants such as caffeine. The sleep environment may be improved by installing heavy drapes to shut out all sunlight, and keeping computers, televisions, and work materials out of the sleeping area.

A 2010 review of published scientific research suggested that exercise generally improves sleep for most people, and helps sleep disorders such as insomnia. The optimum time to exercise may be 4 to 8 hours before bedtime, though exercise at any time of day is beneficial, with the exception of heavy exercise taken shortly before bedtime, which may disturb sleep. However, there is insufficient evidence to draw detailed conclusions about the relationship between exercise and sleep. [94] Sleeping medications such as Ambien and Lunesta are an increasingly popular treatment for insomnia. Although these nonbenzodiazepine medications are generally believed to be better and safer than earlier generations of sedatives, they have still generated some controversy and discussion regarding side effects. White noise appears to be a promising treatment for insomnia. [95]

Obstructive sleep apnea

Obstructive sleep apnea is a condition in which major pauses in breathing occur during sleep, disrupting the normal progression of sleep and often causing other more severe health problems. Apneas occur when the muscles around the patient's airway relax during sleep, causing the airway to collapse and block the intake of oxygen. [96] Obstructive sleep apnea is more common than central sleep apnea. [97] As oxygen levels in the blood drop, the patient then comes out of deep sleep in order to resume breathing. When several of these episodes occur per hour, sleep apnea rises to a level of seriousness that may require treatment.

Diagnosing sleep apnea usually requires a professional sleep study performed in a sleep clinic, because the episodes of wakefulness caused by the disorder are extremely brief and patients usually do not remember experiencing them. Instead, many patients simply feel tired after getting several hours of sleep and have no idea why. Major risk factors for sleep apnea include chronic fatigue, old age, obesity, and snoring.

Aging and sleep

People over age 60 with prolonged sleep (8-10 hours or more average sleep duration of 7-8 hours in the elderly) have a 33% increased risk of all-cause mortality and 43% increased risk of cardiovascular diseases, while those with short sleep (less than 7 hours) have a 6% increased risk of all-cause mortality. [98] Sleep disorders, including sleep apnea, insomnia, or periodic limb movements, occur more commonly in the elderly, each possibly impacting sleep quality and duration. [98] A 2017 review indicated that older adults do not need less sleep, but rather have an impaired ability to obtain their sleep needs, and may be able to deal with sleepiness better than younger adults. [99] Various practices are recommended to mitigate sleep disturbances in the elderly, such as having a light bedtime snack, avoidance of caffeine, daytime naps, excessive evening stimulation, and tobacco products, and using regular bedtime and wake schedules. [100]

Other disorders

Sleep disorders include narcolepsy, periodic limb movement disorder (PLMD), restless leg syndrome (RLS), upper airway resistance syndrome (UARS), and the circadian rhythm sleep disorders. Fatal familial insomnia, or FFI, an extremely rare genetic disease with no known treatment or cure, is characterized by increasing insomnia as one of its symptoms ultimately sufferers of the disease stop sleeping entirely, before dying of the disease. [101]

Somnambulism, known as sleepwalking, is a sleeping disorder, especially among children. [102]

Low quality sleep has been linked with health conditions like cardiovascular disease, obesity, and mental illness. While poor sleep is common among those with cardiovascular disease, some research indicates that poor sleep can be a contributing cause. Short sleep duration of less than seven hours is correlated with coronary heart disease and increased risk of death from coronary heart disease. Sleep duration greater than nine hours is also correlated with coronary heart disease, as well as stroke and cardiovascular events. [103]

In both children and adults, short sleep duration is associated with an increased risk of obesity, with various studies reporting an increased risk of 45–55%. Other aspects of sleep health have been associated with obesity, including daytime napping, sleep timing, the variability of sleep timing, and low sleep efficiency. However, sleep duration is the most-studied for its impact on obesity. [103]

Sleep problems have been frequently viewed as a symptom of mental illness rather than a causative factor. However, a growing body of evidence suggests that they are both a cause and a symptom of mental illness. Insomnia is a significant predictor of major depressive disorder a meta-analysis of 170,000 people showed that insomnia at the beginning of a study period indicated a more than the twofold increased risk for major depressive disorder. Some studies have also indicated correlation between insomnia and anxiety, post-traumatic stress disorder, and suicide. Sleep disorders can increase the risk of psychosis and worsen the severity of psychotic episodes. [103]

Drugs which induce sleep, known as hypnotics, include benzodiazepines, although these interfere with REM [104] Nonbenzodiazepine hypnotics such as eszopiclone (Lunesta), zaleplon (Sonata), and zolpidem (Ambien) Antihistamines, such as diphenhydramine (Benadryl) and doxylamine Alcohol (ethanol), despite its rebound effect later in the night and interference with REM [104] [105] barbiturates, which have the same problem melatonin, a component of the circadian clock, and released naturally at night by the pineal gland [106] and cannabis, which may also interfere with REM. [107]

Stimulants, which inhibit sleep, include caffeine, an adenosine antagonist amphetamine, MDMA, empathogen-entactogens, and related drugs cocaine, which can alter the circadian rhythm, [108] [109] and methylphenidate, which acts similarly and other analeptic drugs like modafinil and armodafinil with poorly understood mechanisms.

Dietary and nutritional choices may affect sleep duration and quality. One 2016 review indicated that a high-carbohydrate diet promoted a shorter onset to sleep and a longer duration of sleep than a high-fat diet. [110] A 2012 investigation indicated that mixed micronutrients and macronutrients are needed to promote quality sleep. [111] A varied diet containing fresh fruits and vegetables, low saturated fat, and whole grains may be optimal for individuals seeking to improve sleep quality. [110] High-quality clinical trials on long-term dietary practices are needed to better define the influence of diet on sleep quality. [110]

Anthropology

Research suggests that sleep patterns vary significantly across cultures. [112] [113] The most striking differences are observed between societies that have plentiful sources of artificial light and ones that do not. [112] The primary difference appears to be that pre-light cultures have more broken-up sleep patterns. [112] For example, people without artificial light might go to sleep far sooner after the sun sets, but then wake up several times throughout the night, punctuating their sleep with periods of wakefulness, perhaps lasting several hours. [112]

The boundaries between sleeping and waking are blurred in these societies. [112] Some observers believe that nighttime sleep in these societies is most often split into two main periods, the first characterized primarily by deep sleep and the second by REM sleep. [112]

Some societies display a fragmented sleep pattern in which people sleep at all times of the day and night for shorter periods. In many nomadic or hunter-gatherer societies, people will sleep on and off throughout the day or night depending on what is happening. [112] Plentiful artificial light has been available in the industrialized West since at least the mid-19th century, and sleep patterns have changed significantly everywhere that lighting has been introduced. [112] In general, people sleep in a more concentrated burst through the night, going to sleep much later, although this is not always the case. [112]

Historian A. Roger Ekirch thinks that the traditional pattern of "segmented sleep," as it is called, began to disappear among the urban upper class in Europe in the late 17th century and the change spread over the next 200 years by the 1920s "the idea of a first and second sleep had receded entirely from our social consciousness." [114] [115] Ekirch attributes the change to increases in "street lighting, domestic lighting and a surge in coffee houses," which slowly made nighttime a legitimate time for activity, decreasing the time available for rest. [115] Today in most societies people sleep during the night, but in very hot climates they may sleep during the day. [116] During Ramadan, many Muslims sleep during the day rather than at night. [117]

In some societies, people sleep with at least one other person (sometimes many) or with animals. In other cultures, people rarely sleep with anyone except for an intimate partner. In almost all societies, sleeping partners are strongly regulated by social standards. For example, a person might only sleep with the immediate family, the extended family, a spouse or romantic partner, children, children of a certain age, children of a specific gender, peers of a certain gender, friends, peers of equal social rank, or with no one at all. Sleep may be an actively social time, depending on the sleep groupings, with no constraints on noise or activity. [112]

People sleep in a variety of locations. Some sleep directly on the ground others on a skin or blanket others sleep on platforms or beds. Some sleep with blankets, some with pillows, some with simple headrests, some with no head support. These choices are shaped by a variety of factors, such as climate, protection from predators, housing type, technology, personal preference, and the incidence of pests. [112]

In mythology and literature

Sleep has been seen in culture as similar to death since antiquity [118] in Greek mythology, Hypnos (the god of sleep) and Thanatos (the god of death) were both said to be the children of Nyx (the goddess of night). [118] John Donne, Samuel Taylor Coleridge, Percy Bysshe Shelley, and other poets have all written poems about the relationship between sleep and death. [118] Shelley describes them as "both so passing, strange and wonderful!" [118] Many people consider dying in one's sleep the most peaceful way to die. [118] Phrases such as "big sleep" and "rest in peace" are often used in reference to death, [118] possibly in an effort to lessen its finality. [118] Sleep and dreaming have sometimes been seen as providing the potential for visionary experiences. In medieval Irish tradition, in order to become a filí, the poet was required to undergo a ritual called the imbas forosnai, in which they would enter a mantic, trancelike sleep. [119] [120]

Many cultural stories have been told about people falling asleep for extended periods of time. [121] [122] The earliest of these stories is the ancient Greek legend of Epimenides of Knossos. [121] [123] [124] [125] According to the biographer Diogenes Laërtius, Epimenides was a shepherd on the Greek island of Crete. [121] [126] One day, one of his sheep went missing and he went out to look for it, but became tired and fell asleep in a cave under Mount Ida. [121] [126] When he awoke, he continued searching for the sheep, but could not find it, [121] [126] so he returned to his old farm, only to discover that it was now under new ownership. [121] [126] He went to his hometown, but discovered that nobody there knew him. [121] Finally, he met his younger brother, who was now an old man, [121] [126] and learned that he had been asleep in the cave for fifty-seven years. [121] [126]

A far more famous instance of a "long sleep" today is the Christian legend of the Seven Sleepers of Ephesus, [121] in which seven Christians flee into a cave during pagan times in order to escape persecution, [121] but fall asleep and wake up 360 years later to discover, to their astonishment, that the Roman Empire is now predominantly Christian. [121] The American author Washington Irving's short story "Rip Van Winkle", first published in 1819 in his collection of short stories The Sketch Book of Geoffrey Crayon, Gent., [122] [127] is about a man in colonial America named Rip Van Winkle who falls asleep on one of the Catskill Mountains and wakes up twenty years later after the American Revolution. [122] The story is now considered one of the greatest classics of American literature. [122]

In art

Of the thematic representations of sleep in art, physician and sleep researcher Meir Kryger wrote, "[Artists] have intense fascination with mythology, dreams, religious themes, the parallel between sleep and death, reward, abandonment of conscious control, healing, a depiction of innocence and serenity, and the erotic." [128]


Methods

Participants

Participants (n = 112) comprised adolescents in the Need for Sleep (NFS) 4 15,28 and NFS5 44 studies. They were between 15 and 19 years old, had no history of chronic medical, psychiatric or sleep disorders, a body mass index (BMI) of ≤ 30 kg/m 2 , were not habitually short sleepers (actigraphically assessed TIB ≥ 6 h with weekend sleep extension ≤ 1 h), consumed ≤ 5 caffeinated beverages per day, and did not travel across > 2 time zones one month prior to the study.

Participants in both studies were randomised into continuous and split sleep groups with total sleep opportunity differing between the two studies (6.5 h in NFS4 and 8 h in NFS5), resulting in four groups: 6.5 h-continuous (n = 29), 6.5 h-split (n = 29), 8 h-continuous (n = 29) and 8 h-split (n = 24). Those in the continuous groups were afforded an opportunity to sleep only at night, while those in the split sleep groups had a night sleep opportunity and a 1.5-h afternoon nap. Both studies were registered clinical trials (NCT03333512 and NCT04044885).

Groups did not significantly differ in age, sex, BMI, daily caffeine consumption, morningness-eveningness preference (Morningness Eveningness Questionnaire), levels of excessive daytime sleepiness (Epworth Sleepiness Scale), symptoms of chronic sleep reduction (Chronic Sleep Reduction Questionnaire), self-reported sleep quality (Pittsburgh Sleep Quality Index) and actigraphically assessed sleep patterns (p > 0.22 Table 1). Informed consent was obtained from all participants and legal guardians in compliance with a protocol approved by the National University of Singapore Institutional Review Board. All experiments were performed in accordance with relevant guidelines and regulations. Participants were compensated financially upon completion of the study.

Procedure

The NFS4 and NFS5 study protocols tracked cognitive performance across 15 days (Fig. 1) as part of a summer camp at a local boarding school. Participants first attended a briefing at least two weeks prior to commencement of the study where an actiwatch was provided for a one week period in order to assess habitual sleep (Table 1). Subsequently, participants also wore an actiwatch in the one week period immediately prior to commencement of the summer camp protocols. This confirmed that all participants refrained from napping and adhered to a 9 h sleep schedule (23:00–08:00), and were therefore well-rested at the beginning of the camp.

The protocol was conducted in a boarding school. Both protocols began with two baseline nights of 9 h TIB followed by two cycles of sleep manipulation and recovery (9 h TIB). The first cycle consisted of five nights of manipulation (M11–M15) followed by two nights of recovery (R11–R12), while the second cycle involved three nights of manipulation (M21–M23) followed by two nights of recovery (R21–R22). The second week was shorter purely for logistic reasons. On manipulation nights, the 6.5 h-continuous schedule included 6.5 h nocturnal TIB (00:15–06:45), while the 6.5 h-split schedule included 5 h nocturnal TIB (01:00–06:00) followed by a 1.5 h nap opportunity (14:00–15:30). The 8 h-continuous schedule included 8 h nocturnal TIB (23:30–07:30) while the 8 h-split schedule included 6.5 h nocturnal TIB (00:15–06:45) and a 1.5 h daytime nap opportunity (14:00–15:30). Thus, nocturnal sleep episodes were anchored to mid-sleep at 03:30 for all sleep schedules. This provided bedtimes and wake times that were practicable, while minimizing the confounding influence of evening or morning light exposure on circadian phase. Sleep–wake patterns were monitored throughout the protocol using actigraphy. Polysomnography (PSG) was recorded on nine nights (B1, B2, M11, M13, M15, R11, M21, M23, and R21), and daytime naps were monitored with PSG on five manipulation days (M11, M13, M15, M21, and M23). Split and continuous groups were housed on different floors. All participants slept in twin-share, air-conditioned rooms. Participants were allowed to adjust their room temperature according to their own comfort. Bedroom windows were fitted with blackout panels to prevent participants from being awoken by sunlight.

All cognitive tasks were programmed in E-Prime 2.0 (Psychology Software Tools, Inc., Sharpsburg, PA) and were administered via individual laptops in a classroom setting. Participants were required to wear earphones to minimize distraction. To avoid circadian confounds, timing of tasks was identical for all groups. Picture encoding took place on M15, and retrieval after two recovery nights on R12, both at 16:45. Learning sessions for the factual knowledge task took place during the second week of sleep manipulation (M21–M23) with the morning session at 11:00 and the afternoon session at 16:45. The factual knowledge retrieval took place after one recovery night on R21 at 20:30. This evening test ensured that the circadian context during retrieval would not favour retrieval of materials learned in either the morning or afternoon. The difference in length of recovery periods between learning and testing of the two memory tasks was for camp scheduling reasons. There is little evidence that sleep deprivation impairs retrieval processes themselves 8 , therefore a one night recovery period was deemed to be sufficient.

Apart from cognitive testing periods, participants spent their time in a common room that received natural as well as electrical lighting. During free periods, participants were allowed to play board games, read, and use their own electronic devices. They were allowed to interact freely with research staff and the other participants. Strenuous physical activities and the consumption of caffeine were prohibited. Activities and cognitive tasks were similar across NFS studies. No additional memory tasks occurred in the period between learning and test sessions for our two reported memory tasks in order to minimise interference effects. All participants were constantly monitored by the research staff.

Picture-encoding task

The task consisted of 240 coloured scene images presented centrally on a computer screen one at a time. Half of the images depicted scenes containing buildings while the remaining half depicted landscapes with no buildings. The images were split into 3 sets of 80 images (40 buildings and 40 no-buildings). Two sets (160 images) were presented during both the encoding and retrieval sessions while the last remaining set (80 images) served as foil images to be presented during retrieval. Image sets used during encoding and retrieval sessions were counterbalanced across participants.

The encoding session took place in a single 15-min block. Participants were instructed to indicate whether or not the image contained a building. They were not informed that their memory for the images would be tested later. The images were presented for 2500 ms each, followed by a response screen showing ‘(1) building, (2) no building’. After having made a response with the corresponding keypress, an inter-trial interval of 1000 ms followed before the start of a new trial. The order of presentation for the images was randomized.

The retrieval session tested the participants’ recognition of 160 ‘old’ images presented during encoding randomly intermixed with 80 ‘new’ foil images. A five-point confidence scale was presented below the images: ‘(1) definitely did not see, (2) probably did not see, (3) unsure, (4) probably saw, (5) definitely saw’. Participants were instructed to indicate whether or not they remembered seeing the image from the previous session. Trials ended once a response was made or after a time limit of 5000 ms, then followed by an inter-trial interval of 1000 ms.

Responses were recorded for the encoding and retrieval sessions. Trials with incorrect responses during encoding were excluded from retrieval analyses as they indicate that images were not adequately attended to. Responses from the retrieval session were categorized into four outcome measures: (1) ‘hits’ included confidence ratings of 4 (probably saw) and 5 (definitely saw) to old images, (2) ‘false alarms’ included confidence ratings of 4 and 5 to new images, (3) ‘misses’ included ratings of 1 (definitely did not see) and 2 (probably did not see) to old images, and (4) ‘correct rejections’ included ratings of 1 and 2 to new images. To account for participants’ response bias toward old/new responses, the non-parametric signal detection measure A’ was calculated (Supplementary Eq. (S1)) where 0.5 indicates chance performance 52 .

Factual knowledge task—pretest

The pretest was administered before learning began to gauge prior knowledge of the species to-be-learned. This involved five stages: (1) picture identification: participants identified the name of each species (two options) from an image. (2) General knowledge: 20 two-alternative forced choice questions about general characteristics of amphibians. (3) Specific knowledge: 20 two-alternative forced choice questions on to-be-learned material, similar to questions that would be administered in the final retrieval test. (4) Subjective disgust: participants rated the amount of disgust they felt toward each species on a scale of 1–9 (no disgust to extreme disgust), to control for the influence of emotion on memory. (5) Subjective knowledge: participants rated the amount of prior knowledge they had for each species on a scale of 1–9 (no knowledge to extensive knowledge). All questions were self-paced and presented in a random order.

Factual knowledge task—encoding

Participants learned factual information about 12 species of amphibians consisting of different animal types: three frogs (Poison Dart Frog, Flying Frog, Gray Tree Frog), three toads (Burrowing Toad, Yellow-bellied Toad, Cane Toad), three newts (Alpine Newt, Orange-bellied Newt, Great Crested Newt), and three salamanders (Giant Salamander, Green Salamander, Mud Puppy). Information regarding the characteristics of the species were adapted from their actual biology and behaviours. At the start of the session, participants were informed that all of the information they learned would be tested at a later date with example test questions shown using different species. Additionally, participants were instructed not to discuss or look up information about amphibians outside of the learning sessions.

Learning took place over 3 days. Each day participants underwent two learning blocks in the morning and two blocks in the afternoon, with each block focusing on a different type of animal (e.g., newts). Learning blocks lasted 30 min each. Participants always learned the same pairs of animal in the morning (e.g., newts and frogs) and afternoon (e.g., toads and salamanders) with the order of learning these pairs switched each day. Animal type was counterbalanced across participants for the morning and afternoon sessions. This ensured that information learned in the morning and afternoon was comparable. The repetition of learning over 3 consecutive days was intended to examine the interaction between memory and sleep in an ecologically valid way. It mimicked the way students learn information on separate occasions spaced in time, while also accounting for the effect of repeated cycles of sleep on learning. A single nap or nocturnal sleep episode can inform about how sleep impacts on memory encoding or consolidation, but it is necessary to examine several sleep episodes and learning opportunities together in order to understand the overall impact of daily sleep patterns on long-term memory.

The learning materials consisted of approximately 80 slides of factual information for each animal type presented in the form of numbered points and images. Participants were able to go through the slides at their own pace (moving forwards and backwards freely), but they were advised to keep to a certain pace to ensure that all slides were seen. To facilitate this, a timer was visible throughout the learning session with slides having markers indicating how much time should have passed in 5-min intervals.

To assist with learning, some slides asked participants to write down on a piece of paper what they could recall about the information learned in the previous slides. Participants were also encouraged to take notes which were returned to the experimenter at the end of each block. The final slide of each block instructed participants to use the remaining time to revise the information learned.

At the end of each block participants completed the Karolinska Sleepiness Scale (KSS) and were asked to rate three questions on a 7-point scale: “Was your attention focused on the task or something unrelated to the task? (1 = completely on task, 7 = completely off task)”, “How motivated were you to learn the information? (1 = completely motivated, 7 = completely unmotivated)”, and “How well do you feel you could learn the information? (1 = extremely well, 7 = extremely poorly)”. These scales measured subjective ‘focus’, ‘motivation’, and ‘ability’ respectively. Scores were subsequently inverted for analysis so that higher values represented higher levels for each measure (Supplementary Fig. S1, Table S1).

Factual knowledge task—retrieval

Questions were presented in a two-alternative forced choice format followed by a confidence rating (certain, somewhat certain, guess). The foil was usually the answer to the same question for a different species. Analysis focussed on participants most confident responses (those rated as “certain”) because these were less likely to be contaminated with noise introduced by guessing 2,10,15 . These were corrected for response bias by subtracting incorrect from correct responses. Questions and confidence ratings remained on screen until participants gave a response or after 10 s had elapsed. In total there were 360 questions (90 questions for each animal type). Half of the questions were related to the material learned in the morning and the other half in the afternoon. Questions were presented randomly in six blocks separated by 30 s breaks. Participants were instructed to think carefully about their responses within the given time limit.

Actigraphy

Participants wore an Actiwatch AW-2 (Phillips Respironics Inc., Pittsburgh, PA) during three different periods: (1) 1 week of screening, (2) 1 week immediately before the camp, and (3) two weeks during the camp. Sleep diaries were used to clean and verify the data. Data were scored with Actiware software (version 6.0.2) at 30-s resolution and medium sensitivity.

Polysomnography

Electroencephalography (EEG) was performed via a SOMNOtouch recorder (SOMNOmedics GmbH, Randersacker, Germany) on two channels (C3 and C4) in the international 10–20 system), referenced to contralateral mastoids. Cz and Fpz were used as common reference and ground electrodes respectively. EEG electrode impedances were kept < 5 kΩ and electrooculography (EOG) and submental electromyography (EMG) impedances were kept < 10 kΩ. Pulse oximetry was measured on the first night (B1) to assess for undiagnosed sleep apnoea.

EEG signal was sampled at 256 Hz and band-pass filtered between 0.2 and 35 Hz for EEG, and between 0.2 and 10 Hz for EOG. Automated scoring of sleep stages and artefacts were performed using Z3Score 53 (https://z3score.com) in conjunction with the FASST EEG toolbox, and was also visually checked by trained staff following criteria from the American Academy of Sleep Medicine Manual for the Scoring of Sleep and Associated Events 23 .

For each recording, the following parameters were computed: TST, N2 latency (time from lights off to N2 onset), durations of N1, N2, N3 and REM sleep, as well as sleep efficiency and WASO. As a measure of homeostatic sleep pressure, we computed slow wave energy (SWE 0.6–4 Hz) in the first hour of sleep from N2 onset by integrating power in the delta band summed across all NREM epochs, and normalized to SWE in the first hour of B2 41 . Automatic sleep spindle detection analysis was performed using the Wonambi Python package, v5.24 (https://wonambi-python.github.io) with an automated algorithm 54 . Spindle count was computed for NREM epochs using C3-A2 electrodes.

Statistical analysis

For picture encoding we assessed accuracy during encoding and memory at retrieval (A’) via a 2 × 2 ANOVA with between-subject factors of sleep schedule (split/continuous) and duration (6.5 h/8 h).

For the factual knowledge task, we assessed prior knowledge of the stimuli via one-way ANOVA on test scores for general knowledge, specific knowledge, picture identification, as well as subjective knowledge and disgust. Certain memory (correct-incorrect) during retrieval was analysed with a 2 × 2 × 2 mixed ANOVA including schedule (split/continuous), duration (6.5 h/8 h), and the within-subject factor of time (morning/afternoon). Paired and independent sample t-tests were used for follow-up comparisons. Subjective measures (alertness, focus, ability, motivation) were assessed via the same 2 × 2 × 2 mixed ANOVA.

For PSG, total sleep time and duration spent in sleep stages across 24 h were compared using a one-way ANOVA, and independent t-tests. Pearson’s correlations were performed between memory and sleep measures: picture encoding used the total sleep period 24 h prior to encoding (M15) and recovery sleep shortly after encoding (R11). Factual knowledge learning took place from M21 to M23, therefore we averaged the PSG-assessed sleep parameters obtained across M21 and M23.

All effect sizes were computed with Cohen’s d (d) for t-tests of key comparisons. All statistical tests were two-tailed with significance level of p < 0.05 (uncorrected).


Contents

Polyphasic sleep can be caused by irregular sleep-wake syndrome, a rare circadian rhythm sleep disorder which is usually caused by neurological abnormality, head injury or dementia. [3] Much more common examples are the sleep of human infants and of many animals. Elderly humans often have disturbed sleep, including polyphasic sleep. [4]

In their 2006 paper "The Nature of Spontaneous Sleep Across Adulthood", [5] Campbell and Murphy studied sleep timing and quality in young, middle-aged, and older adults. They found that, in free-running conditions, the average duration of major nighttime sleep was significantly longer in young adults than in the other groups. The paper states further:

Whether such patterns are simply a response to the relatively static experimental conditions, or whether they more accurately reflect the natural organization of the human sleep/wake system, compared with that which is exhibited in daily life, is open to debate. However, the comparative literature strongly suggests that shorter, polyphasically-placed sleep is the rule, rather than the exception, across the entire animal kingdom (Campbell and Tobler, 1984 Tobler, 1989). There is little reason to believe that the human sleep/wake system would evolve in a fundamentally different manner. That people often do not exhibit such sleep organization in daily life merely suggests that humans have the capacity (often with the aid of stimulants such as caffeine or increased physical activity) to overcome the propensity for sleep when it is desirable, or is required, to do so.

One classic cultural example of a biphasic sleep pattern is the practice of siesta, which is a nap taken in the early afternoon, often after the midday meal. Such a period of sleep is a common tradition in some countries, particularly those where the weather is warm. The siesta is historically common throughout the Mediterranean and Southern Europe. It is the traditional daytime sleep of China, [6] India, South Africa, Italy, [7] Spain and, through Spanish influence, the Philippines and many Hispanic American countries.

A separate biphasic sleep pattern is sometimes described as segmented sleep, often consisting of going to sleep early at night, awakening in the post-midnight hours, and then returning to bed for a second period of sleep into the morning. The New York Times asserts that this practice was common in the past -- "in the preindustrial West, most people slept in two discrete blocks." [8] Benjamin Franklin was a prominent example of this sleeping pattern. [8]

Interrupted sleep is a primarily biphasic sleep pattern where two periods of nighttime sleep are punctuated by a period of wakefulness. Along with a nap in the day, it has been argued that this is the natural pattern of human sleep in long winter nights. [9] [10] A case has been made that maintaining such a sleep pattern may be important in regulating stress. [10]

As historical norm Edit

Historian A. Roger Ekirch [11] [12] has argued that before the Industrial Revolution, interrupted sleep was dominant in Western civilization. He draws evidence from more than 500 references to a segmented sleeping pattern in documents from the ancient, medieval, and modern world. [10] Other historians, such as Craig Koslofsky, [13] have endorsed Ekirch's analysis.

According to Ekirch's argument, adults typically slept in two distinct phases, bridged by an intervening period of wakefulness of approximately one hour. [12] This time was used to pray [14] and reflect, [15] and to interpret dreams, which were more vivid at that hour than upon waking in the morning. This was also a favorite time for scholars and poets to write uninterrupted, whereas still others visited neighbors, engaged in sexual activity, or committed petty crime. [12] : 311–323

The human circadian rhythm regulates the human sleep-wake cycle of wakefulness during the day and sleep at night. Ekirch suggests that it is due to the modern use of electric lighting that most modern humans do not practice interrupted sleep, which is a concern for some writers. [16] Superimposed on this basic rhythm is a secondary one of light sleep in the early afternoon.

The brain exhibits high levels of the pituitary hormone prolactin during the period of nighttime wakefulness, which may contribute to the feeling of peace that many people associate with it. [17]

The modern assumption that consolidated sleep with no awakenings is the normal and correct way for human adults to sleep, may lead people to consult their doctors fearing they have maintenance insomnia or other sleep disorders. [10] If Ekirch's hypothesis is correct, their concerns might best be addressed by reassurance that their sleep conforms to historically natural sleep patterns. [18]

Ekirch has found that the two periods of night sleep were called "first sleep" (occasionally "dead sleep") and "second sleep" (or "morning sleep") in medieval England. He found that first and second sleep were also the terms in the Romance languages, as well as in the language of the Tiv of Nigeria. In French, the common term was premier sommeil or premier somme in Italian, primo sonno in Latin, primo somno or concubia nocte. [12] : 301–302 He found no common word in English for the period of wakefulness between, apart from paraphrases such as first waking or when one wakes from his first sleep and the generic watch in its old meaning of being awake. In old French an equivalent generic term is dorveille, a portmanteau of the French words dormir (to sleep) and veiller (to be awake).

Because members of modern industrialised societies, with later evening hours facilitated by electric lighting, mostly do not practice interrupted sleep, Ekirch suggests that they may have misinterpreted and mistranslated references to it in literature. Common modern interpretations of the term first sleep are "beauty sleep" and "early slumber". A reference to first sleep in the Odyssey was translated as "first sleep" in the seventeenth century, but, if Ekirch's hypothesis is correct, was universally mistranslated in the twentieth. [12] : 303

In his 1992 study "In short photoperiods, human sleep is biphasic", Thomas Wehr had seven healthy men confined to a room for fourteen hours of darkness daily for a month. At first the participants slept for about eleven hours, presumably making up for their sleep debt. After this the subjects began to sleep much as people in pre-industrial times were claimed to have done. They would sleep for about four hours, wake up for two to three hours, then go back to bed for another four hours. They also took about two hours to fall asleep. [9]

In crises and other extreme conditions, people may not be able to achieve the recommended eight hours of sleep per day. Systematic napping may be considered necessary in such situations.

Claudio Stampi, as a result of his interest in long-distance solo boat racing, has studied the systematic timing of short naps as a means of ensuring optimal performance in situations where extreme sleep deprivation is inevitable, but he does not advocate ultrashort napping as a lifestyle. [19] Scientific American Frontiers (PBS) has reported on Stampi's 49-day experiment where a young man napped for a total of three hours per day. It purportedly shows that all stages of sleep were included. [20] Stampi has written about his research in his book Why We Nap: Evolution, Chronobiology, and Functions of Polyphasic and Ultrashort Sleep (1992). [21] In 1989 he published results of a field study in the journal Work & Stress, concluding that "polyphasic sleep strategies improve prolonged sustained performance" under continuous work situations. [22] In addition, other long-distance solo sailors have documented their techniques for maximizing wake time on the open seas. One account documents the process by which a solo sailor broke his sleep into between 6 and 7 naps per day. The naps would not be placed equiphasically, instead occurring more densely during night hours. [23]

U.S. military Edit

The U.S. military has studied fatigue countermeasures. An Air Force report states:

Each individual nap should be long enough to provide at least 45 continuous minutes of sleep, although longer naps (2 hours) are better. In general, the shorter each individual nap is, the more frequent the naps should be (the objective remains to acquire a daily total of 8 hours of sleep). [24]

Canadian Marine pilots Edit

Similarly, the Canadian Marine pilots in their trainer's handbook report that:

Under extreme circumstances where sleep cannot be achieved continuously, research on napping shows that 10- to 20-minute naps at regular intervals during the day can help relieve some of the sleep deprivation and thus maintain . performance for several days. However, researchers caution that levels of performance achieved using ultrashort sleep (short naps) to temporarily replace normal sleep are always well below that achieved when fully rested. [25]

NASA Edit

NASA, in cooperation with the National Space Biomedical Research Institute, has funded research on napping. Despite NASA recommendations that astronauts sleep eight hours a day when in space, they usually have trouble sleeping eight hours at a stretch, so the agency needs to know about the optimal length, timing and effect of naps. Professor David Dinges of the University of Pennsylvania School of Medicine led research in a laboratory setting on sleep schedules which combined various amounts of "anchor sleep", ranging from about four to eight hours in length, with no nap or daily naps of up to 2.5 hours. Longer naps were found to be better, with some cognitive functions benefiting more from napping than others. Vigilance and basic alertness benefited the least while working memory benefited greatly. Naps in the individual subjects' biological daytime worked well, but naps in their nighttime were followed by much greater sleep inertia lasting up to an hour. [26]

Italian Air Force Edit

The Italian Air Force (Aeronautica Militare Italiana) also conducted experiments for their pilots. In schedules involving night shifts and fragmentation of duty periods through the entire day, a sort of polyphasic sleeping schedule was studied. Subjects were to perform two hours of activity followed by four hours of rest (sleep allowed), this was repeated four times throughout the 24-hour day. Subjects adopted a schedule of sleeping only during the final three rest periods in linearly increasing duration. The AMI published findings that "total sleep time was substantially reduced as compared to the usual 7–8 hour monophasic nocturnal sleep" while "maintaining good levels of vigilance as shown by the virtual absence of EEG microsleeps." EEG microsleeps are measurable and usually unnoticeable bursts of sleep in the brain while a subject appears to be awake. Nocturnal sleepers who sleep poorly may be heavily bombarded with microsleeps during waking hours, limiting focus and attention. [27]

There is an active community that experiments with alternative sleeping schedules to achieve more time awake each day, but the effectiveness of this is disputed. [28]

Researcher Piotr Woźniak argues that the theory behind severe reduction of total sleep time by way of short naps is unsound, and that there is no brain control mechanism that would make it possible to adapt to the "multiple naps" system. Woźniak expresses concern that the ways in which the polyphasic sleepers' attempt to limit total sleep time, restrict time spent in the various stages of the sleep cycle, and disrupt their circadian rhythms will eventually cause them to suffer the same negative effects as those with other forms of sleep deprivation or circadian rhythm sleep disorder. Woźniak claims to have scanned the blogs of polyphasic sleepers and found that they have to choose an "engaging activity" again and again just to stay awake and that polyphasic sleep does not improve one's learning ability or creativity. [29]

There are many claims [ citation needed ] that polyphasic sleep was used by polymaths and prominent people such as Leonardo da Vinci, Napoleon, and Nikola Tesla, but there are few if any reliable sources confirming these. One first person account comes from Buckminster Fuller, who described a regimen consisting of 30-minute naps every six hours. The short article about Fuller's nap schedule in Time in 1943, which referred to the schedule as "intermittent sleeping", says that he maintained it for two years, and notes that "he had to quit because his schedule conflicted with that of his business associates, who insisted on sleeping like other men." [30]


Sleep Deprivation, Psychosis and Mental Efficiency

Today, average young adults report sleeping about seven to seven and one-half hours each night. Compare this to sleep patterns in 1910, before the electric lightbulb, the average person slept nine hours each night. This means that today's population sleeps one to two hours less than people did early in the century.

Today, average young adults report sleeping about seven to seven and one-half hours each night. Compare this to sleep patterns in 1910, before the electric lightbulb, the average person slept nine hours each night. This means that today's population sleeps one to two hours less than people did early in the century (Webb and Agnew, 1975).

Because of the advent of the lightbulb, people sleep 500 hours less each year than they used to. Unfortunately, our current "sleep diet" is significantly less than evolution intended. Most other primates (e.g., apes and monkeys) have a 24-hour sleep and activity cycle that is similar to that of humans who live in cultures where the siesta is still practiced. These animals have a long sleep at night, and a shorter sleep in the midafternoon, with a daily sleep total of about 10 hours. Humans seem to naturally need about the same amount of sleep. For instance, when the pressure of work, alarm clocks, social schedules and advanced technology is removed, people tend to sleep longer. Thus, in many less industrialized societies, the total daily sleep time is still around nine to 10 hours as it is for people when they are on unstructured holidays (Coren, 1996a).

Confirmation of these natural sleep durations comes from Palinkas, Suedfeld and Steel (1995). These researchers spent a summer above the arctic circle where there is continuous light 24 hours a day. All watches, clocks and other timekeeping devices were removed, and only the station's computers tracked the times that the team went to sleep and awakened. Individual researchers did their work, and chose when to sleep or wake according to their "body time." At the end of the experiment, they found that their overall average sleep daily time was 10.3 hours. Every member of the team showed an increase in sleep time, with the shortest logging in at 8.8 hours a day, and the longest almost 12 hours a day. This study, like many others, seems to suggest that our biological need for sleep might be closer to the 10 hours per day that is typical of monkeys and apes living in the wild, than the 7 to 7.5 hours typical of humans in today's high-tech, clock-driven lifestyle.

Psychological researchers have tended to minimize the effects of sleep insufficiency, acknowledging that society may be getting too little sleep, but treating the effects of this sleep deprivation as nothing more significant than an inconvenience which makes people feel a bit tired now and then.

This view is incorrect. Recent research suggests that each day with insufficient sleep increases our sleep debt and, when this sleep debt becomes large enough, noticeable problems appear (Coren, 1996a). These sleep-debt-related problems are most predictable at certain times of the day. This is because the efficiency of our physical and mental functions show cyclic increases and decreases in the form of circadian rhythms. While our major sleep/wakefulness rhythm has a cycle length of roughly 24 hours, there are shorter cycles as well, with the most important of these being a secondary sleep/wakefulness cycle that is around 12 hours.

Because of these cycles, the pressure to fall asleep is greatest in the morning, between 1 and 4 a.m. In addition there is a less pronounced, but still noticeable, increase in sleepiness 12 hours later, between 1 and 4 p.m. It is this afternoon low point that makes you feel sleepy after lunch, not the meal that you may have just eaten. It probably also was the original reason for the afternoon nap or siesta.

People who are operating with a sleep debt are less efficient, and this inefficiency is most noticeable when the circadian cycle is at its lowest ebb. Among the common consequences of a large sleep debt are attentional lapses, reduced short-term memory capacity, impaired judgment and the occurrence of "microsleeps."

A microsleep is a short period of time, usually between 10 seconds to a minute in length, in which the brain actually enters a sleep state, regardless of what the person is doing at the time. The affected individual often does not know that this momentary blackout has occurred. The effects of these microsleeps combined with attentional lapses, however, can be dramatic.

There is now evidence that many major disasters have been due to sleep-debt related effects. The evidence shows that these include the oil spill of the Exxon Valdez, the nuclear accidents at Chernobyl and Three Mile Island, and the loss of the space shuttle Challenger (Coren, 1996a).

In fact, our societal sleep debt is so great that simply losing one additional hour of sleep due to the spring shift to daylight savings time can increase traffic accident rates by 7% (Coren, 1996b) and death rates due to all accidents by 6.5% (Coren, 1996c).

When sleep deprivation becomes great enough, the effects mimic those of psychosis. The failure of the scientific world to recognize this is due to some extent to the folklore that has grown up around the sleepless marathon of high school student Randy Gardner in 1964.

To gain an entry into the Guiness Book of World Records, he remained awake for 264 hours (11 days). Summaries of this case usually report that Gardner suffered no hallucinations, no paranoia or other negative mood changes, and that his mental, motor and sensory abilities were quite good throughout the entire episode. This conclusion is so widespread that it has now become a stock "fact" presented in virtually any psychology or psychiatry book that has a chapter on sleep.

This conclusion seems to be based on two items of information. The first was the observation that there were no obvious lasting physical or mental problems encountered by Gardner when he began to sleep again. The second was based upon observations of researcher William Dement (Dement, 1992), who interviewed Gardner on Day 10 of the experiment. He reported that he took Gardner to a restaurant and then played pinball with him, noting that Gardner played the game well and even won. Lt. Cmdr. John J. Ross of the U.S. Navy Medical Neuropsychiatric Research Unit in San Diego, who was called in by Gardner's worried parents to monitor his condition, tells a quite different story (Ross, 1965). Gardner's symptoms that Ross reported included:

  • Day 2: Difficulty focusing eyes and signs of astereognosis (difficulty recognizing objects only by touch).
  • Day 3: Moodiness, some signs of ataxia (inability to repeat simple tongue twisters).
  • Day 4: Irritability and uncooperative attitude, memory lapses and difficulty concentrating. Gardner's first hallucination was that a street sign was a person, followed by a delusional episode in which he imagined that he was a famous black football player.
  • Day 5: More hallucinations (e.g., seeing a path extending from the room in front of him down through a quiet forest). These were sometimes described as "hypnagogic reveries" since Gardner recognized, at least after a short while, that the visions were illusionary in nature.
  • Day 6: Speech slowing and difficulty naming common objects.
  • Day 7 and 8: Irritability, speech slurring and increased memory lapses.
  • Day 9: Episodes of fragmented thinking frequently beginning, but not finishing, his sentences.
  • Day 10: Paranoia focused on a radio show host who Gardner felt was trying to make him appear foolish because he ws having difficulty remembering some details about his vigil.
  • Day 11: Expressionless appearance, speech slurred and without intonation had to be encouraged to talk to get him to respond at all. His attention span was very short and his mental abilities were diminished. In a serial sevens test, where the respondent starts with the number 100 and proceeds downward by subtracting seven each time, Gardner got back to 65 (only five subtractions) and then stopped. When asked why he had stopped he claimed that he couldn't remember what he was supposed to be doing.

In many respects Gardner's symptoms were similar to those experienced by a New York disk jockey, Peter Tripp, who endured a 200-hour sleepless marathon to raise money for the March of Dimes. During the course of his ordeal his thoughts became increasingly distorted and there were marked periods of irrationality. By the end of four days he could not successfully execute simple tests requiring focused attention. In addition, he began to have hallucinations and distorted visual perceptions. At one point Tripp became quite upset when he thought that the spots on a table were insects. He thought that there were spiders crawling around the booth and even once complained that they had spun cobwebs on his shoes.

He showed the same increasing moodiness and paranoia that Gardner did. On his last day, a neurologist was called to examine Tripp before sending him home. When Tripp looked up at this doctor in his dark, old-fashioned suit, he had the delusion that the doctor was really an undertaker who was about to bury him alive. Overtaken with fear, he let loose a scream and bolted for the door. Half-dressed, Tripp ran down the hall with doctors and psychologists in pursuit. He could no longer distinguish the difference between reality and nightmare.

This same pattern of mental deterioration that mimicks psychotic symptoms appears in several more systematic studies of sleep deprivation and extreme sleep debt. Thus, prolonged sleep deprivation does lead to the appearance of serious mental symptoms. In addition, even moderate amounts of sleep deprivation can lead to losses in mental efficiency that can threaten public and personal safety.

(According to a 1995 report by the U.S. Department of Transportation, 100,000 sleep-related traffic accidents claim some 1,500 American lives each year. The National Commission on Sleep Disorders Research has reported that sleep-related accidents, and sleep disorders which impact work productivity, cost the American economy between $100 and $150 billion each year-Ed.)

References:

References

Coren S (1996a), Sleep Thieves. New York: Free Press.

Coren S (1996b), Daylight savings time and traffic accidents. New Eng J Med 334:924.

Coren S (1996c), Accidental death and the shift to daylight savings time. Percept Mot Skills 83:921-922.

Dement WC (1992), The Sleepwatchers. Stanford, Calif.: Stanford Alumni Association.

Palinkas LA, Suedfeld P, Steel GD (1995), Psychological functioning among members of a small polar expedition. Avia Space Environ Med 66:943-950.

Ross JJ (1965), Neurological findings after prolonged sleep deprivation. Arch Neurol 12:399-403.
Webb WB, Agnew HW (1975), Are we chronically sleep deprived? Bull the Psychonomic Soc 6:47-48.


Work or sleep? Honeybee foragers opportunistically nap during the day when forage is not available

Shifts in work schedules test humans’ capacity to be flexible in the timing of both work and sleep. Honeybee, Apis mellifera, foragers also shift their work schedules, but how flexible they are in the timing of sleep as they shift the timing of work is unknown, despite the importance of colony-level plasticity in the face of a changing environment. We hypothesized that sleep schedules of foragers are not fixed and instead vary depending on the time when food is available. We trained bees to visit a food source made available for several hours in the early morning (AM) or several hours in the late afternoon (PM), then monitored their sleep behaviour for 24 h after training, specifically comparing their sleep during the AM and PM periods previously designated as training periods. Following AM training, honeybee foragers slept more during the afternoon than during the morning, but following PM training, the same bees ‘slept in’ the next morning, and so slept more in the morning than in the afternoon. Although foragers did not change the total amount of time devoted to each of their behaviours (including sleep), the timing of their sleep did change. Thus, plasticity in timing of foraging was matched by plasticity in timing of sleep. The apparent correlation between the timing patterns of foraging and sleeping demonstrates temporal plasticity of sleep under ecologically realistic conditions in an invertebrate. Testing how shift work affects the health and performance of honeybees may shed light on the role of sleep in a nonhuman social animal.


Conclusions

Disturbed sleep is a core symptom of depression and its normalization is necessary to achieve remission from the illness. In the long term, all antidepressants which show clinical efficacy improve sleep secondary to improvement of mood and daytime activity. However, in the short term, while some of them may impair sleep due to the activating effects, other may improve sleep due to the sedative properties. Although sleep-promoting action is desired in depressed patients with coexisting anxiety or insomnia, it may be problematic during the maintenance treatment after recovery from depression due to oversedation. Thus, it is necessary to understand the effects of these drugs on the sleep and daytime alertness. It is particularly noteworthy that for sleep-promoting effect, it is sufficient to use a sedative antidepressant in a low dose. In such dose, these drugs can be also combined with other antidepressants as an alternative to hypnotic drugs, especially if there is a clinical necessity to promote sleep for longer than 2𠄴 weeks with a frequency higher than 3𠄴 times per week.