Sleep

For other uses, see Sleep (disambiguation).

Sleep is the natural state of rest observed throughout the animal kingdom, in all mammals and birds, and in many reptiles, amphibians, and fish.

In humans, other mammals, and a substantial majority of other animals which have been studied by humans — such as fish, birds, ants, and fruit-flies — regular sleep is necessary for survival. While sleep is essential for survival, there currently does not exist credible scientific evidence behind the purpose of sleep.

Physiology (Stages of Sleep)

In mammals, the measurement of eye movement during sleep is used to divide sleep into two broad types:

• rapid eye movement (REM) and
• non-rapid eye movement (NREM) sleep.

Each type has a distinct set of associated physiological, neurological and psychological features.

Sleep proceeds in cycles of REM and NREM phases. In humans, this cycle is approximately 90 to 120 minutes. Each phase may have a distinct physiological function. Drugs such as alcohol and sleeping pills can suppress certain stages of sleep (see Sleep deprivation). This can result in a sleep that exhibits loss of consciousness but does not fulfill its physiological functions.

Rechtschaffen and Kales originally outlined the criteria for staging sleep in 1968. The American Academy of Sleep Medicine (AASM) updated the staging rules in 2007.

Stage 4 Sleep. EEG highlighted by red box.

REM Sleep. EEG highlighted by red box. Eye movements highlighted by red line.

Non-REM (NREM) Sleep

Further information: Non-rapid eye movement

NREM accounts for 75–80% of total sleep time in normal human adults. In NREM sleep, the body is active and the brain is inactive, and there is relatively little dreaming. Non-REM encompasses four stages; stages 1 and 2 are considered 'light sleep', and 3 and 4 'deep sleep'. They are differentiated solely using EEG, unlike REM sleep which is characterized by rapid eye movements and relative absence of muscle tone. There are often limb movements, and parasomnia sleep walking occurs in non-REM sleep. A cyclical alternating pattern may sometimes be observed during a stage.

NREM consists of four stages according to the 2007 AASM standards:

• During Stage N1 the brain transitions from alpha waves (common to people who are awake and having a frequency of 8 to 13 Hz) to theta waves (frequency of 4 to 7 Hz). This stage is sometimes referred to as somnolence, or "drowsy sleep". Associated with the onset of sleep during N1 may be sudden twitches and hypnic jerks. These are normal, as is an increased instance of flatulence. Some people may also experience hypnagogic hallucinations during this stage, which can be more troublesome to the subject. During N1 the subject loses some muscle tone, and conscious awareness of the external environment.
• Stage N2, is characterized by "sleep spindles" (12 to 16 Hz) and "K-complexes." During this stage the electromyography (EMG) lowers, and conscious awareness of the external environment disappears. This occupies 45 to 55% of total sleep.
• In Stage N3, the delta waves, also called delta rhythms (0.5 to 4 Hz) make up less than 50% of the total wave-patterns. This is considered part of the slow-wave sleep (SWS) and functions primarily as a transition into stage N4. This is the stage in which night terrors, bed wetting, sleepwalking, and sleep-talking occur.
• In Stage N4, delta-waves make up more than 50% of the wave-patterns. Stages N3 and N4 are the deepest forms of sleep; N4 is effectively a deeper version of N3, in which the deep-sleep characteristics, such as delta-waves, are more pronounced.

Both REM sleep and NREM sleep stages 3 and 4 are homeostatically driven; that is, if a human is selectively deprived of one of these, it rebounds once the person is allowed to sleep. This suggests that both are essential in the sleep process and its many functions.

Functions

Despite decades of research, knowledge of sleep's function is minimal. Several theories and axioms are listed herein.

Restoration

Sleep affects the body in several ways. Wound healing has been shown to be affected by sleep. A study conducted by Gumustekin et al. shows sleep deprivation hindering the healing of burns on rats. The subjects were 50 male rats and they all received full-skin-thickness burns. They were then divided in 5 groups of 10 but only two groups are important to the claim, they are the control and sleep deprived group. The experimental group of rats was then sleep deprived for 72 hours by switching wood shavings in the cages with water. After the sleep deprivation period, rats were evaluated to determine the extent of wound healing that occurred. A scale with scores from 0 to 3, no cells to countless cells, was used to evaluate the amount of polymorphonuclear leucocytes (PNLs, a type of white blood cell), immunoglobulin G (IgGs, a type of antibody), fibroblasts (a cell that maintains tissue and play an important role in wound healing), and proliferation of capillaries in samples ^[3]. When compared to the control, there was a score decrease of 78% in fibroblasts, an 89% score decrease in capillaries, a 92% score increase in PNLs levels, and a 34% score increase in IgG levels. These changes in wound healing from sleep deprivation indicate that sleep is related to healing. It’s also been shown that sleep deprivation affects the immune system and metabolism. In a study by Zager, Andersen, Ruiz, Antunes, and Tufik 10 ninety day old rats were deprived of sleep for 24 hours. When compared with a control group the sleep deprived rat’s blood tests indicated a 20% decrease in white blood cell count, which is a significant change in the immune system. Another study, this one conducted by Bonnet and Arand , indicates that sleep affects metabolism. Eighteen human subjects were chosen for this study and paired by sex, age, and weight, 4 females and 14 males, but one of the two in the pair had a diagnosed form of insomnia called sleep state misperception, and the other member was a normal sleeper. All subjects stayed at the lab for two nights and had standard clinical polysomnograms performed on them. Metabolic rate was measured on the second night alone. The researchers defined metabolic rate in their study as a measure of overall oxygen use (VO₂ measured in mL min^-1), as they felt it was the best indicator of physiological activity. It was found that the insomniacs slept only 6.8 hours a night to the normal sleepers' 7.5 hours. The overall metabolism mean value for the insomniacs was 286 mL min^-1 where as the normal sleepers mean value was 304 mL min^-1, a significant difference.

It has yet to be clearly proven that sleep affects somatic growth. One study by Jenni, Molinari, Caflisch, and Largo recorded growth, height and weight, as it correlates to sleep in 305 children. These children were volunteered by their parents and information was collected from age 1 to 10. The subject’s guardians were interviewed at regular intervals to obtain information on their child’s sleep duration. Jenni et al. ^[6] found the mean sleep duration of the subjects decreased with age by about 8% per year while mean height increased by 4% and mean weight increased by 14% per year. It has been shown that sleep, more specifically slow-wave sleep (SWS), does affect growth hormone levels. A study by Van Cauter, Leproult, and Plat used 149 healthy men as subjects ages 16 to 83. Blood samples were taken for growth hormone measurement every 15 to 30 minutes for 24 hours. At night subjects were allowed to sleep for 8 hours and all-night polygraphic sleep recordings were obtained. The results showed that subjects with high SWS (average SWS percentage of 24%) also had high growth hormone secretion, an average of 275µg. Subjects with low SWS (average SWS percentage of 9%) also had low growth hormone secretion, an average of 150µg.

Anabolic/catabolic cycle

Non-REM sleep may be an anabolic state marked by physiological processes of growth and rejuvenation of the organism's immune, nervous, muscular, and skeletal systems (but see above). Wakefulness may perhaps be viewed as a cyclical, temporary, hyperactive catabolic state during which the organism acquires nourishment and procreates.

Ontogenesis

According to the ontogenetic hypothesis of REM sleep, the activity occurring during neonatal REM sleep (or active sleep) seems to be particularly important to the developing organism (Marks et al., 1995). Studies investigating the effects of deprivation of active sleep have shown that deprivation early in life can result in behavioral problems, permanent sleep disruption, decreased brain mass (Mirmiran et al. 1983), and an abnormal amount of neuronal cell death (Morrissey, Duntley & Anch, 2004).

Memory processing

Over the years scientists have shown numerous ways in which sleep is related to memory. In a study conducted by Turner, Drummond, Salamat, and Brown working memory was shown to be affected by sleep deprivation. Working memory is important because it keeps information active for further processing and supports higher-level cognitive functions such as decision making, reasoning, and episodic memory ^[8]. Turner et al. ^[8] allowed 18 women and 22 men to sleep only 26 minutes per night over a 4 day period. Subjects were given initial cognitive tests while well rested and then tested again twice a day during the 4 days of sleep deprivation. On the final test the average working memory span of the sleep deprived group dropped by 38% in comparison to the control group. This demonstrates that there is clearly a connection between sleep and memory.

Memory also seems to be affected differently by certain stages of sleep such as rapid eye movement sleep (REM) and slow-wave sleep (SWS). One study citied in Born, Rasch, and Gais took subjects and made multiple groups; wake control groups and sleep test groups. Sleep and wake groups were taught a task and then tested on it both on early and late nights, with the order of nights balanced across participants. When the subject’s brains were scanned during sleep, hypnograms revealed that SWS was the dominant sleep stage during the early night representing around 23% on average for sleep stage activity. The early night test group performed 16% better on the declarative memory test than the control group. During late night sleep, REM became the most active sleep stage at about 24%, and the late night test group performed 25% better on the procedural memory test than the control group. This indicates that procedural memory benefits from late REM-rich sleep where as declarative memory benefits from early SWS-rich sleep.^[9]

Another study conducted by Datta indirectly supports these results. The subjects chosen were 22 male rats. A box was constructed where a single rat could move freely from one end to the other. The bottom of the box was made of a steel grate. A light would shine in the box accompanied by a sound. After a 5 second delay an electrical shock would be applied. Once the shock commenced the rat could move to the other end of the box ending the shock immediately. The rat could also use the 5 second delay to move to the other end of the box and avoid the shock entirely. The length of the shock never exceeded 5 seconds. This was repeated 30 times for half the rats. The other half, the control group, was placed in the same trial but the rats were shocked regardless of their reaction. After each of the training sessions the rat would be placed in a recording cage for 6 hours of polygraphic recordings. This process was repeated for 3 consecutive days. This study found that during the post-trial sleep recording session rats spent 25.47% more time in REM sleep after learning trials than after control trials.^[10] These trials support the results of the Born et al. ^[9] study, indicating an obvious correlation between REM sleep and procedural knowledge.

Another interesting observation of the Datta ^[10] study is that the learning group spent 180.57% more time in SWS than did the control group during the post-trial sleep-recording session. This phenomenon is supported by a study performed by Kudrimonti, Barnes, and McNaughton . This study shows that after spatial exploration activity patterns of hippocampal place cells are reactivated during SWS following the experiment. In a study by Kudrimonti et al. ^[11] seven rats were run through a linear track using rewards on either end. The rats would then be placed in the track for 30 minutes to allow them to adjust (PRE), then they ran the track with reward based training for 30 minutes (RUN), and then they were allowed to rest for 30 minutes. During each of these three periods EEG data was collected for information on the rats’ sleep stages. Kudrimonti et al. ^[11] computed the mean firing rates of hippocampal place cells during pre-behavior SWS (PRE) and three 10 min intervals in post-behavior SWS (POST) by averaging across 22 track-running sessions from seven rats. The results showed that 10 min after the trial RUN session there was a 12% increase in the mean firing rate of hippocampal place cells from the PRE level, however after 20 minutes the mean firing rate returned rapidly toward the PRE level ^[11]. The elevated firing of hippocampal place cells during SWS after spatial exploration could explain why there were elevated levels of SWS sleep in Datta’s ^[10] study as it also dealt with a form of spatial exploration.

The different studies all suggest that there is a correlation between sleep and the many complex functions of memory.

Preservation

The "Preservation and Protection" theory holds that sleep serves an adaptive function. It protects the person during that portion of the 24-hour day in which being awake, and hence roaming around, would place the individual at greatest risk. Organisms do not require 24 hours to feed themselves and meet other necessities. From this perspective of adaptation, organisms are safer by staying out of harm's way where potentially they could be prey to other, stronger organisms. They sleep at times that maximize their safety, given their physical capacities and their habitats. (Allison & Cicchetti, 1976; Webb, 1982).

However, this theory fails to explain why the brain disengages from the external environment during normal sleep. A seemingly more advantageous adaptation for animals would be to seclude themselves but remain quietly awake to avoid predation. In fact, animals who are preyed upon usually disengage from the external environment to a lesser degree. Another argument against the theory is that sleep is not simply a passive consequence of removing the animal from the environment, but is a "drive": animals alter their behaviors in order to obtain sleep. Therefore, circadian regulation is more than sufficient to explain periods of activity and quiescence that are adaptive to an organism, but the more peculiar specializations of sleep probably serve different and unknown functions.

These theories are not mutually exclusive; each may contain truths that may be validated in the future. Recent studies show that sleep is phylogenetically ancient (Shaw et al Science 2000, Hendricks et al Neuron 2000). Thus, to understand the function of sleep, we must study simple animals that predated the arthropoda and chordata phyla, as well as the roles of proteins and enzymes in basic metabolism. Some sleep features are unique to mammals (e.g. REM sleep and thermoregulation) and thus probably did not occur in sleeplike states of primordial metazoa.

Optimal amount

Adults

The optimal amount of sleep may be equally important to the timing of that sleep in relation to an individual's circadian rhythms. A person's major sleep episode is relatively inefficient and inadequate when it occurs at the "wrong" time of day. The timing is correct when the circadian markers maximum concentration of the hormone melatonin and the minimum core body temperature occur near the beginning of the second half of the sleep episode.

The National Sleep Foundation maintains that eight to nine hours of sleep for adult humans is optimal and that sufficient sleep benefits alertness, memory and problem solving, and overall health, as well as reducing the risk of accidents. A widely publicized 2003 study performed at the University of Pennsylvania School of Medicine demonstrated that cognitive performance declines with fewer than eight hours of sleep.

A University of California, San Diego psychiatry study found that people who live the longest sleep for six to seven hours each night. However, this study cannot be used to determine optimal sleep habits, only correlation — and empirically observed correlation is a necessary but not sufficient condition for causality. For example, such correlation can be explained from the fact that older people tend to sleep less, or perhaps a genetic ability to generate cells faster provides advantages in both sleep necessity and longevity.

Researchers from the University of Warwick, and University College London have found that lack of sleep can more than double the risk of death from cardiovascular disease, but that too much sleep can also double the risk of death . Professor Francesco Cappuccio said: “Short sleep has been shown to be a risk factor for weight gain, hypertension and Type 2 diabetes sometimes leading to mortality but in contrast to the short sleep-mortality association it appears that no potential mechanisms by which long sleep could be associated with increased mortality have yet been investigated. Some candidate causes for this include depression, low socioeconomic status and cancer-related fatigue.” “In terms of prevention, our findings indicate that consistently sleeping around 7 hours per night is optimal for health and a sustained reduction may predispose to ill-health.”

Children

A child sleeping

Children need a greater amount of sleep than adults to function correctly (up to 18 hours for newborn babies, with a declining rate as the child ages).

Age	Average total number of hours sleeping per day
Newborn	18
1 month	15–16
3 months	15
6 months	14–15
9 months	14
1 year	13–14
2 years	13
3 years	12
4 years	11 1/2
5 years	11
6 years	11
7 years	10
8 years	10
9 years	9-10
10-17 years	8-9.5

Longest period without sleep

Depending on how one defines sleep, there are several people who can claim the record for having gone the longest without sleep.

1. Thai Ngoc, born 1942, has been awake for 33 years or 11,700 nights, according to Vietnamese news organization Thanh Nien. It was said that Ngoc acquired the ability to go without sleep after a bout of fever in 1973, but other reports indicate he stopped sleeping in 1976 with no known trigger. At the time of the Thanh Nien report, Ngoc suffered from no apparent ill effect (other than a minor decline in liver function), was mentally sound and could carry 100 kg of pig feed down a 4 km road, but another report indicates that he was healthy before the sleepless episode but that now he was not feeling well because of the lack of sleep.
2. Randy Gardner holds the Guinness World Record for intentionally having gone the longest without sleep. In 1965, Gardner, then 18, stayed awake for 264 hours (about 11 days) for a high school science project. He experienced significant deficits in concentration, motivation, perception and other higher mental processes during his sleep deprivation. However, he recovered normal cognitive functions after a few nights' sleep.
3. On May 25 2007 the BBC reported that Tony Wright beat the Guinness World Record by staying awake for 11 days and nights. The Guinness Book of Records has, however, withdrawn its backing of a sleep deprivation class because of the associated health risks.
4. People born with the rare genetic disorder Morvan’s fibrillary chorea or Morvan's syndrome can go without sleep for several months at a time. Michel Jouvet and his colleagues in Lyon, France, studied a 27-year-old man and found he had virtually no sleep over a period of several months. During that time he did not feel sleepy or tired and did not show any disorders of mood, memory, or anxiety. Nevertheless, nearly every night between 9:00 and 11:00 p.m., he experienced a 20 to 60-minute period of auditory, visual, olfactory, and somesthetic (sense of touch) hallucinations, as well as pain and vasoconstriction in his fingers and toes. In recent investigations, Morvan's syndrome has been attributed to serum antibodies directed against specific potassium ion (K+) channels in cell and nerve membranes.
5. There have been various, though largely unsubstantiated, claims by "tweakers" (methamphetamine addicts) that with the aid of methamphetamine they have gone without sleep for periods of up to 2 weeks. Users reportedly experience severe auditory and visual hallucinations that descend rapidly into methamphetamine psychosis.

Causes of difficulty sleeping

Many people have trouble sleeping, which may stem from a number of issues, including:

• uncomfortable sleep furnishings
• stress from family, job and/or personal issues
• environmental conditions (excessive heat, cold, pollution, noise, bright light, loud noises)
• environmental surroundings (tidiness of room, odors, cleanliness of room)
• poor body positioning
• illness
• pain
• medicine and drugs (Some medications may cause insomnia, or result in dependency on a drug to fall alseep; others, including recreational drugs, are stimulants that may make sleep difficult or impossible)
• improper sleep timing, as sleep is easier to achieve if one takes one's chronotype into consideration.

A study by researchers at the University of Pennsylvania has confirmed that the more one works, the less one sleeps - and that work is the single biggest factor troubling sleep.

Dreaming

Main article: Dream

Dreaming is the perception of sensory images during sleep, in a sequence which the sleeper/dreamer usually perceives more as an apparent participant than an observer. Dreaming is stimulated by the pons and mostly occurs during the REM phase of sleep.

People have proposed many hypotheses about the functions of dreaming. Sigmund Freud postulated that dreams are the symbolic expression of frustrated desires that had been relegated to the subconscious, and he used dream interpretation in the form of psychoanalysis to uncover these desires. Scientists have become skeptical about the Freudian interpretation, and place more emphasis on dreaming as a requirement for organization and consolidation of recent memory and experience. See Freud:The Interpretation of Dreams

James Allan Hobson's and Robert McCarley's activation synthesis theory proposes that dreams are caused by the random firing of neurons in the cerebral cortex during the REM period. According to the theory, the forebrain then creates a story in an attempt to reconcile and make sense of the nonsensical sensory information presented to it, hence the odd nature of many dreams.

A wet dream is the ejaculation of semen during sleep. This is most often experienced by pubescent boys during REM sleep, but may occur at any time after puberty.

Anthropology of sleep

Pattern

Recent research suggests that sleep patterns vary significantly across human cultures. The most striking differences are between societies that have plentiful artificial light and ones that do not. Cultures without artificial light have more broken-up sleep patterns. This is called segmented sleep, which has led to expressions such as "first sleep," "watch," and "second sleep" which appear in literature from all over the world.

Some cultures have fragmented sleep patterns in which people sleep at all times of the day, and for shorter periods at night. For example, many Mediterranean and Latin American cultures have a siesta, in which people sleep for a period in the afternoon. In many nomadic or hunter-gatherer societies people sleep off and on throughout the day or night depending on what is happening.

Some sleep deprivation-oriented sleep patterns have also been discovered recently, such as that of the Uberman's sleep schedule, which revolve around the theory of sleeping in regular patterns of 20 minute sleep and 4 hours awake time, leading to the body's ability to jump instantly into REM. The effect of this is that the body accumulates a total of 2 hours of REM sleep, whereas a body on a normal sleep schedule accumulates only 1.5 hours of REM.

Since plentiful artificial light became available in some cultures in the mid-19th century, sleep patterns have changed significantly in these cultures. These people sleep in a concentrated burst at night, and sleep later in the morning.

In non-humans

Sleeping Japanese Macaques

A sleeping Komodo Dragon

A sleeping Cat

Cattle, horses, and sheep can sleep while standing or while lying down; however, they cannot experience REM sleep while standing. If deprived of REM sleep for a long time, the animal may involuntarily collapse in order to reach REM sleep, a condition not to be confused with narcolepsy. Whales and dolphins are also different from humans: they always have to be conscious, as they are conscious breathers, so only one half of their brain sleeps at a time. Sleep becomes difficult to define in lower order animals, such as the bullfrog. Its resting state is too similar to its active state to be considered by many to satisfy the criteria for sleep, but brain activity in the resting state is similar to other amphibians that do meet the criteria when they sleep.

See also

Sleep physiology

• Chronotype
• Circadian rhythm
• Hypersomnia
• Insomnia
• Jet lag
• Myoclonic twitch
• Narcolepsy
• Polysomnogram
• Seasonal affective disorder
• Sleep inertia
• Sleep paralysis

Patterns and disruptions

• Hibernation
• Polyphasic sleep
• Segmented sleep
• Sleep debt
• Sleep deprivation
• Unihemispheric slow-wave sleep

Practices and rituals

• Bedtime
• Co-sleeping
• Hypnosis
• Lucid dreaming
• Meditation
• Sleep hygiene
• Yoga Nidra

Other

• Alarm clock
• Dream interpretation
• Dream world (plot device)
• Lullaby
• Sudden infant death syndrome

Further reading

• Bar-Yam, Yaneer (2003). "Chapter 3". Dynamics of Complex Systems. {{cite book}}: |format= requires |url= (help); External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help)
• Foldvary-Schaefer N, Grigg-Damberger M (2006). "Sleep and epilepsy: what we know, don't know, and need to know". J Clin Neurophysiol. 23 (1): 4–20. PMID 16514348. {{cite journal}}: Unknown parameter |month= ignored (help)
• Gilmartin G, Thomas R (2004). "Mechanisms of arousal from sleep and their consequences". Curr Opin Pulm Med. 10 (6): 468–74. PMID 15510052. {{cite journal}}: Unknown parameter |month= ignored (help) [Review]
• Gottlieb D, Punjabi N, Newman A, Resnick H, Redline S, Baldwin C, Nieto F (2005). "Association of sleep time with diabetes mellitus and impaired glucose tolerance". Arch Intern Med. 165 (8): 863–7. PMID 15851636. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
• Legramante J, Galante A (2005). "Sleep and hypertension: a challenge for the autonomic regulation of the cardiovascular system". Circulation. 112 (6): 786–8. PMID 16087808. {{cite journal}}: Unknown parameter |month= ignored (help) [Editorial]
• Feinberg I. Changes in sleep cycle patterns with age J Psychiatr Res. 1974;10:283–306. [review]
• Tamar Shochat and Sonia Ancoli - Specific Clinical Patterns in Aging - Sleep and Sleep Disorders [website]
• Zepelin H. Normal age related changes in sleep. In: Chase M, Weitzman ED, eds. Sleep Disorders: Basic and Clinical Research. New York: SP Medical; 1983:431–434.
• Morrissey M, Duntley S, Anch A, Nonneman R (2004). "Active sleep and its role in the prevention of apoptosis in the developing brain". Med Hypotheses. 62 (6): 876–9. PMID 15142640.{{cite journal}}: CS1 maint: multiple names: authors list (link)
• Marks G, Shaffery J, Oksenberg A, Speciale S, Roffwarg H (1995). "A functional role for REM sleep in brain maturation". Behav Brain Res. 69 (1–2): 1–11. PMID 7546299. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
• Mirmiran M, Scholtens J, van de Poll N, Uylings H, van der Gugten J, Boer G (1983). "Effects of experimental suppression of active (REM) sleep during early development upon adult brain and behavior in the rat". Brain Res. 283 (2–3): 277–86. PMID 6850353. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
• Zhang, J. (2004). "[Memory process and the function of sleep]" (PDF). Journal of Theoretics. 6 (6). {{cite journal}}: Unknown parameter |month= ignored (help)

External links

• American Academy of Sleep Medicine
• Sleep Research Society
• National Sleep Foundation
• National Center on Sleep Disorders Research