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[[File:|thumb|300px|Sleeping baby cat]] , Iran]] Sleep is a natural state of bodily rest observed in humans and other animals. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and it is more easily reversible than hibernation or coma. It is common to all mammals and birds, and is also seen in many reptiles, amphibians, and fish. In humans, other mammals, and a substantial majority of other animals that have been studied (such as some species of fish, birds, ants, and fruit flies), regular sleep is essential for survival. The purposes of sleep are only partly clear and are the subject of intense research.[1]

Contents

Physiology

Stages of sleep

In mammals and birds, sleep is divided into two broad types: Rapid Eye Movement (REM) and Non-Rapid Eye Movement (NREM or non-REM) sleep. Each type has a distinct set of associated physiological, neurological, and psychological features. The American Academy of Sleep Medicine (AASM) further divides NREM into three stages: N1, N2, and N3, the last of which is also called delta, or slow-wave, sleep (SWS).[2] [[File:|thumb|Sleep cycles through the night, with deep sleep early on and more REM (marked in red) toward morning.]]

File:Sleep EEG Stage
Stage N3 sleep; EEG highlighted by red box. Thirty seconds of deep sleep with greater than 50% delta waves.
EEG highlighted by red box; eye movements highlighted by red line. Thirty seconds of sleep.]]

Sleep proceeds in cycles of REM and NREM, the order normally being N1 → N2 → N3 → N2 → REM. There is a greater amount of deep sleep (stage N3) early in the night, while the proportion of REM sleep increases later in the night and just before natural awakening.

The stages of sleep were first described in 1937 by Alfred Lee Loomis and coworkers, who separated the different EEG features of sleep into five levels (A to E), which represented the spectrum of wakefulness to deep sleep.[3] In 1953, REM sleep was discovered as distinct, and thus William Dement and Nathaniel Kleitman reclassified sleep into four NREM stages and REM.[4] The staging criteria were standardized in 1968 by Allan Rechtschaffen and Anthony Kales in the "R&K sleep scoring manual."[5] In the R&K standard, NREM sleep was divided into four stages, with slow-wave sleep comprising stages 3 and 4. In stage 3, delta waves made up less than 50% of the total wave patterns, while they made up more than 50% in stage 4. Furthermore, REM sleep was sometimes referred to as stage 5.

In 2004, the AASM commissioned the AASM Visual Scoring Task Force to review the R&K scoring system. The review culminated in several changes, the most significant being the combination of stages 3 and 4 into Stage N3. The revised scoring was published in 2007 as The AASM Manual for the Scoring of Sleep and Associated Events.[6] Arousals and respiratory, cardiac, and movement events were also added.[7][8]

Sleep stages and other characteristics of sleep are commonly assessed by polysomnography in a specialized sleep laboratory. Measurements taken include electroencephalography (EEG) of brain waves, electrooculography (EOG) of eye movements, and electromyography (EMG) of skeletal muscle activity. In humans, each sleep cycle lasts from 90 to 110 minutes on average,[9] and each stage may have a distinct physiological function. Drugs such as sleeping pills and alcoholic beverages can suppress certain stages of sleep, leading to sleep deprivation[citation needed]. This can result in sleep that exhibits loss of consciousness but does not fulfill its physiological functions (i.e., one may still feel tired after apparently sufficient sleep). REM and slow-wave sleep are both homeostatically driven; people and most animals selectively deprived of one of these stages will rebound once uninhibited sleep is allowed.[citation needed] This finding suggests that both of these stages are essential.

NREM sleep

According to the 2007 AASM standards, NREM consists of three stages. There is relatively little dreaming in NREM.

Stage N1 refers to the transition of the brain from alpha waves having a frequency of 8 to 13 Hz (common in the awake state) to theta waves having a frequency of 4 to 7 Hz. This stage is sometimes referred to as somnolence or drowsy sleep. Sudden twitches and hypnic jerks, also known as positive myoclonus, may be associated with the onset of sleep during N1. Some people may also experience hypnagogic hallucinations during this stage, which can be troublesome to them. During N1, the subject loses some muscle tone and most conscious awareness of the external environment.

Stage N2 is characterized by sleep spindles ranging from 12 to 16 Hz and K-complexes. During this stage, muscular activity as measured by EMG decreases, and conscious awareness of the external environment disappears. This stage occupies 45 to 55% of total sleep in adults.

Stage N3 (deep or slow-wave sleep) is characterized by delta waves ranging from 0.5 to 4 Hz (also called delta rhythms). This is the stage in which such parasomnias as night terrors, bedwetting, sleepwalking, and sleep-talking occur.

REM sleep

Rapid eye movement sleep, or REM sleep, accounts for 20–25% of total sleep time in normal human adults. The criteria for REM sleep include rapid eye movements as well as a rapid low-voltage EEG. Most memorable dreaming occurs in this stage. At least in mammals, a descending muscular atonia is seen. Such paralysis may be necessary to protect organisms from self-damage through physically acting out scenes from the often-vivid dreams that occur during this stage.

Timing

Sleep timing is controlled by the circadian clock, sleep-wake homeostasis, and in humans, within certain bounds, willed behavior. The circadian clock—an inner timekeeping, temperature-fluctuating, enzyme-controlling device—works in tandem with adenosine, a neurotransmitter that inhibits many of the bodily processes associated with wakefulness. Adenosine is created over the course of the day; high levels of adenosine lead to sleepiness. In diurnal animals, sleepiness occurs as the circadian element causes the release of the hormone melatonin and a gradual decrease in core body temperature. The timing is affected by one's chronotype. It is the circadian rhythm that determines the ideal timing of a correctly structured and restorative sleep episode.[10]

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.[11] Along with corresponding messages from the circadian clock, this tells the body it needs to sleep.[12] Sleep offset (awakening) is primarily determined by circadian rhythm. A person who regularly awakens at an early hour will generally not be able to sleep much later than the person's normal waking time, even if moderately sleep-deprived.

Optimal amount in humans

Adult

The optimal amount of sleep is not a meaningful concept unless the timing of that sleep is seen 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; one should be asleep at least six hours before the lowest body temperature.[13] The timing is correct when the following two circadian markers occur after the middle of the sleep episode and before awakening:[14]

  • maximum concentration of the hormone melatonin, and
  • minimum core body temperature.

The National Sleep Foundation in the United States maintains that seven to nine hours of sleep for adult humans is optimal and that sufficient sleep benefits alertness, memory, problem solving, and overall health, as well as reducing the risk of accidents.[15] A widely publicized 2003 study[16] performed at the University of Pennsylvania School of Medicine demonstrated that cognitive performance declines with six or fewer hours of sleep.

A University of California, San Diego, psychiatry study of more than one million adults found that people who live the longest self-report sleeping for six to seven hours each night.[17] Another study of sleep duration and mortality risk in women showed similar results.[18] Other studies show that "sleeping more than 7 to 8 hours per day has been consistently associated with increased mortality," though this study suggests the cause is probably other factors such as depression and socioeconomic status, which would correlate statistically.[19] It has been suggested that the correlation between lower sleep hours and reduced morbidity only occurs with those who wake after less sleep naturally, rather than those who use an alarm.

sleeping.]]

Researchers at 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.[20][21] 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 seven hours per night is optimal for health, and a sustained reduction may predispose to ill health."

Furthermore, sleep difficulties are closely associated with psychiatric disorders such as depression, alcoholism, and bipolar disorder. Up to 90% of patients with depression are found to have sleep difficulties.[citation needed]

Hours by age

sleeping.]]

Children need more sleep per day than adults to develop and function properly: up to 18 hours for newborn babies, with a declining rate as a child ages.[12][15] A newborn baby spends almost 9 hours a day in REM sleep. By the age of five or so, only slightly over two hours is spent in REM.[22]

Age and condition Average amount of sleep per day
Newborn up to 18 hours
1–12 months 14–18 hours
1–3 years 12–15 hours
3–5 years 11–13 hours
5–12 years 9–11 hours
Adolescents 9–10 hours[23]
Adults, including elderly 7–8 (+) hours
Pregnant women 8 (+) hours

Sleep debt

Sleep debt is the effect of not getting enough rest and sleep; a large debt causes mental, emotional, and physical fatigue. It is unclear why a lack of sleep causes irritability; however, theories are emerging that suggest if the body produces insufficient cortisol during deep sleep, it can have negative effects on the alertness and emotions of a person during the day.[citation needed]

Sleep debt results in diminished abilities to perform high-level cognitive functions. Neurophysiological and functional imaging studies have demonstrated that frontal regions of the brain are particularly responsive to homeostatic sleep pressure.[24]

Scientists do not agree on how much sleep debt it is possible to accumulate; whether it is accumulated against an individual's average sleep or some other benchmark; nor on whether the prevalence of sleep debt among adults has changed appreciably in the industrialized world in recent decades. It is likely that children are sleeping less than previously in Western societies.[25]

Functions

The multiple theories proposed to explain the function of sleep reflect the as-yet incomplete understanding of the subject.

It is likely that sleep evolved to fulfill some primeval function and has taken over multiple functions over time—just as with, for example, the larynx which today controls the passage of food and air and provides phonation for communicating and other social purposes.

It has been pointed out that, if sleep were not essential, one should be able to find 1) animal species that do not sleep at all, 2) animals that do not need recovery sleep when they stay awake longer than usual, and 3) animals that suffer no serious consequences as a result of lack of sleep. No animals have been found to date that satisfy any of these criteria.[26]

Some of the many proposed functions of sleep are as follows.

File:Effects of sleep
Main health effects of sleep deprivation,[27] indicating impairment of normal maintenance by sleep.

Restoration

Wound healing has been shown to be affected by sleep. A study conducted by Gumustekin et al.[28] in 2004 shows sleep deprivation hindering the healing of burns on rats.

It has been shown that sleep deprivation affects the immune system. In a study by Zager et al. in 2007,[29] rats were deprived of sleep for 24 hours. When compared with a control group, the sleep-deprived rats' blood tests indicated a 20% decrease in white blood cell count, a significant change in the immune system. It is now possible to state that "sleep loss impairs immune function and immune challenge alters sleep," and it has been suggested that mammalian species which invest in longer sleep times are investing in the immune system, as species with the longer sleep times have higher white blood cell counts.[30]

It has yet to be proven that sleep duration affects somatic growth. One study by Jenni et al.[31] in 2007 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." It has been shown that sleep—more specifically, slow-wave sleep (SWS)—does affect growth hormone levels in adult men. During eight hours' sleep, Van Cauter, Leproult, and Plat[32] found that the men with a high percentage of SWS (average 24%) also had high growth hormone secretion, while subjects with a low percentage of SWS (average 9%) had low growth hormone secretion.

There are multiple arguments supporting the restorative function of sleep. We are rested after sleeping, and it is natural to assume that this is a basic purpose of sleep. The metabolic phase during sleep is anabolic; anabolic hormones such as growth hormones (as mentioned above) are secreted preferentially during sleep. The duration of sleep among species is, in general, inversely related to animal size and directly related to basal metabolic rate. Rats with a very high basal metabolic rate sleep for up to 14 hours a day, whereas elephants and giraffes with lower BMRs sleep only 3–4 hours per day.

Energy conservation could as well have been accomplished by resting quiescent without shutting off the organism from the environment, potentially a dangerous situation. A sedentary nonsleeping animal is more likely to survive predators, while still preserving energy. Sleep, therefore, seems to serve another purpose, or other purposes, than simply conserving energy; for example, hibernating animals waking up from hibernation go into rebound sleep because of lack of sleep during the hibernation period. They are definitely well-rested and are conserving energy during hibernation, but need sleep for something else.[33] Rats kept awake indefinitely develop skin lesions, hyperphagia, loss of body mass, hypothermia, and eventually, septicemia and death.[34]

Anabolic/catabolic

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 (with some exceptions). Wakefulness may perhaps be viewed as a cyclical, temporary, hyperactive catabolic state during which the organism acquires nourishment and reproduces.

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).

REM sleep appears to be important for development of the brain. REM sleep occupies the majority of time of sleep of infants, who spend most of their time sleeping. Among different species, the more immature the baby is born, the more time it spends in REM sleep. Proponents also suggest that REM-induced muscle inhibition in the presence of brain activation exists to allow for brain development by activating the synapses, yet without any motor consequences that may get the infant in trouble. Additionally, REM deprivation results in developmental abnormalities later in life.

However, this does not explain why older adults still need REM sleep. Aquatic mammal infants do not have REM sleep in infancy;[35] REM sleep in those animals increases as they age.

Memory processing

Scientists have shown numerous ways in which sleep is related to memory. In a study conducted by Turner, Drummond, Salamat, and Brown,[36] 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. Turner et al. allowed 18 women and 22 men to sleep only 26 minutes per night over a four-day period. Subjects were given initial cognitive tests while well-rested, and then were tested again twice a day during the four days of sleep deprivation. On the final test, the average working memory span of the sleep-deprived group had dropped by 38% in comparison to the control group.

Memory seems to be affected differently by certain stages of sleep such as REM and slow-wave sleep (SWS). In one study cited in Born, Rasch, and Gais,[37] multiple groups of human subjects were used: wake control groups and sleep test groups. Sleep and wake groups were taught a task and were then tested on it, both on early and late nights, with the order of nights balanced across participants. When the subjects' 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, whereas declarative memory benefits from early, SWS-rich sleep.

A study conducted by Datta[38] indirectly supports these results. The subjects chosen were 22 male rats. A box was constructed wherein 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 five-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 five-second delay to move to the other end of the box and avoid the shock entirely. The length of the shock never exceeded five 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 six hours of polygraphic recordings. This process was repeated for three consecutive days. This study found that during the posttrial sleep recording session, rats spent 25.47% more time in REM sleep after learning trials than after control trials. These trials support the results of the Born et al. study, indicating an obvious correlation between REM sleep and procedural knowledge.

An observation of the Datta study is that the learning group spent 180% 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 Kudrimoti, Barnes, and McNaughton.[39] This study shows that after spatial exploration activity, patterns of hippocampal place cells are reactivated during SWS following the experiment. In a study by Kudrimoti et al., 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 were collected for information on the rats' sleep stages. Kudrimoti et al. computed the mean firing rates of hippocampal place cells during prebehavior SWS (PRE) and three ten-minute intervals in postbehavior SWS (POST) by averaging across 22 track-running sessions from seven rats. The results showed that ten minutes 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. 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 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 complex functions of memory. Harvard sleep researchers Saper and Stickgold[40] point out that an essential part of memory and learning consists of nerve cell dendrites' sending information to the cell body to be organized into new neuronal connections. This process demands that no external information is presented to these dendrites, and they suggest that this may be why it is during sleep that we solidify memories and organize knowledge.

Preservation

The "Preservation and Protection" theory holds that sleep serves an adaptive function. It protects the animal 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. 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.

Moreover, the preservation theory does not explain why carnivores like lions, which are on top of the food chain, sleep the most. By the preservation logic, these top carnivores should not need any sleep at all. Preservation also does not explain why aquatic mammals sleep while moving. Lethargy during these vulnerable hours would do the same and would be more advantageous, because the animal will be quiescent but still be able to respond to environmental challenges like predators, etc. Sleep rebound that occurs after a sleepless night will be maladaptive, but still occurs for a reason. For example, a zebra falling asleep the day after it spent the sleeping time running from a lion is more, not less, vulnerable to predation.

Dreaming

Dreaming is the perception of sensory images and sounds during sleep, in a sequence which the 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 unconscious mind, and he used dream interpretation in the form of psychoanalysis to uncover these desires. See Freud: The Interpretation of Dreams.

Freud's work concerns the psychological role of dreams, which clearly does not exclude any physiological role they may have. It is not ruled out therefore by the increased modern interest in the organization and consolidation of recent memory and experience. Recent research claims that sleep has this overall role of consolidation and organization of synaptic connections formed during learning and experience.

Rosalind Cartwright stated, "One function of dreams may be to restore our sense of competence… it is also probable that in many times of stress, dreams have more work to do in resolving our problems and are thus more salient and memorable."[41]

John Allan Hobson 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 this 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.[42]

Effect of food and drink on sleep

Depressants

Often, people start drinking alcohol in order to get to sleep (alcohol is initially a sedative and will cause somnolence, encouraging sleep). [43] However, being addicted to alcohol can lead to disrupted sleep, because alcohol has a rebound effect later in the night. As a result, there is strong evidence linking alcoholism and insomnia.[44]

Barbiturates cause drowsiness and have actions similar to ethanol (drinking alcohol).

Melatonin is a naturally occurring hormone that regulates sleepiness. It is made in the brain, where tryptophan is converted into serotonin and then into melatonin, which is released at night by the pineal gland to induce and maintain sleep. Melatonin supplementation may be used as a sleep aid, both as a hypnotic and as a chronobiotic (see phase response curve, PRC).

  • Siesta and the "post-lunch dip"

Many people have a temporary drop in alertness in the early afternoon, commonly known as the "post-lunch dip." While a large meal can make a person feel sleepy, the post-lunch dip is mostly an effect of the biological clock. People naturally feel most sleepy (have the greatest "drive for sleep") at two times of the day about 12 hours apart—for example, at 2:00 a.m. and 2:00 p.m. At those two times, the body clock "kicks in." At about 2 p.m. (14:00), it overrides the homeostatic buildup of sleep debt, allowing several more hours of wakefulness. At about 2 a.m. (02:00), with the daily sleep debt paid off, it "kicks in" again to ensure a few more hours of sleep.

The amino acid tryptophan is a building block of proteins. It has been claimed to contribute to sleepiness, since it is a precursor of the neurotransmitter serotonin, involved in sleep regulation. However, no solid data have ever linked modest dietary changes in tryptophan to changes in sleep.

Stimulants

Amphetamines (amphetamine, dextroamphetamine, methamphetamine, etc.) are often used to treat narcolepsy and ADHD disorders and are used recreationally, in which case they may be referred to as "speed." Their most common effects are decreased hunger, anxiety, insomnia, stimulation, and increased alertness. Adderall is a mixture of amphetamine salts used to treat ADHD.

Caffeine is a stimulant that works by slowing the action of the hormones in the brain that cause somnolence, particularly by acting as an antagonist at adenosine receptors. Effective dosage is individual, in part dependent on prior usage. It can cause a rapid reduction in alertness as it wears off.

Studies on cocaine have shown its effects to be mediated through the circadian rhythm system.[45] This may be related to the onset of hypersomnia (oversleeping) in regard to "Cocaine-Induced Sleep Disorder."[46]

The stimulating effects of energy drinks come from stimulants such as caffeine, sugars, and essential amino acids, and they will eventually create a rapid reduction in alertness similar to that of caffeine.

The class of drugs called empathogen-entactogens keep users awake with intense euphoria. Commonly known as "ecstasy."

Commonly known by the brand names Ritalin and Concerta, methylphenidate is similar in action to amphetamines and cocaine.

Causes of difficulty in sleeping

There are many reasons for poor sleep. Following sleep hygienic principles may solve problems of physical or emotional discomfort.[47] When pain, illness, drugs, or stress are the culprit, the cause must be treated. Sleep disorders—including the sleep apneas, narcolepsy, primary insomnia, periodic limb movement disorder (PLMD), restless leg syndrome (RLS), and the circadian rhythm sleep disorders—are treatable.

Elderly people may to some degree lose the ability to consolidate sleep. They need the same amount per day as they've always needed, but may need to take some of their sleep as daytime naps.

Anthropology of sleep

Research suggests that sleep patterns vary significantly across cultures.[48][49] The most striking differences are between societies that have plentiful sources of artificial light and ones that do not.[citation needed] The primary difference appears to be that prelight cultures have more broken-up sleep patterns.[48] For example, people 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.[48] The boundaries between sleeping and waking are blurred in these societies.[48] Some observers believe that nighttime sleep in these societies is most often split into two main periods, the first characterised primarily by deep sleep and the second by REM sleep.[citation needed] This segmented sleep has led to expressions such as "first sleep," "watch," and "second sleep," which appear in literature from preindustrial societies all over the world.[citation needed]

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.[50]

Plentiful artificial light has been available in the industrialised West since at least the mid-19th century, and sleep patterns have changed significantly everywhere that lighting has been introduced.[citation needed] In general, people sleep in a more concentrated burst through the night, going to sleep much later, although this is not always true.[citation needed]

In some societies, people generally sleep with at least another person (often many) or with animals. In others, people rarely sleep with anyone but a most intimate relation, such as a spouse. In almost all societies, sleeping partners are strongly regulated by social standards. For example, people might only sleep with their immediate family, extended family, spouses, their children, children of a certain age, children of 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.[50]

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, and the incidence of pests.[48]

Sleep in nonhumans

s.]]

Many animals sleep, but neurological sleep states are difficult to define in lower-order animals. In these animals, sleep is defined using behavioral characteristics such as minimal movement, postures typical for the species, and reduced responsiveness to external stimulation. Sleep is quickly reversible, as opposed to hibernation or coma, and sleep deprivation is followed by longer or deeper sleep. Herbivores, who require a long waking period to gather and consume their diet, typically sleep less each day than similarly sized carnivores, who might well consume several days' supply of meat in a sitting.

Horses and other herbivorous ungulates can sleep while standing, but must necessarily lie down for REM sleep (which causes muscular atony) for short periods. Giraffes, for example, only need to lie down for REM sleep for a few minutes at a time. Bats sleep while hanging upside down. Some aquatic mammals and some birds can sleep with one half of the brain while the other half is awake, so-called unihemispheric slow-wave sleep.[51] Birds and mammals have cycles of non-REM and REM sleep (as described above for humans), though birds’ cycles are much shorter and they do not lose muscle tone (go limp) to the extent that most mammals do.

Many mammals sleep for a large proportion of each 24-hour period when they are very young.[52] However, killer whales and some dolphins do not sleep during the first month of life.[53] Such differences may be explained by the ability of land-mammal newborns to be easily protected by parents while sleeping, while marine animals must, even while very young, be more continuously vigilant for predators.

See also

Positions, practices, and rituals

Other

References

Bibliography

Notes

  1. ^ Bingham, Roger; Terrence Sejnowski, Jerry Siegel, Mark Eric Dyken, Charles Czeisler, Paul Shaw, Ralph Greenspan, Satchin Panda, Philip Low, Robert Stickgold, Sara Mednick, Allan Pack, Luis de Lecea, David Dinges, Dan Kripke, Giulio Tononi (February 2007). "Waking Up To Sleep" (Several conference videos). The Science Network. http://thesciencenetwork.org/programs/waking-up-to-sleep. Retrieved on 2008-01-25. 
  2. ^ Silber, et al. (2007). "The Visual Scoring of Sleep in Adults" (Full text). J Clin Sleep Med (American Academy of Sleep Medicine) 3: 121–31. http://www.aasmnet.org/jcsm/Articles/030203.pdf. Retrieved on 2009-03-04. 
  3. ^ Loomis, Alfred L; Harvey EN, Hobart GA (1937). "III Cerebral states during sleep, as studied by human brain potentials". J Exp Psychol. 21: 127–44. doi:10.1037/h0057431. 
  4. ^ Dement, William; Nathaniel Kleitman (1957). "Cyclic variations in EEG during sleep and their relation to eye movements, body motility and dreaming". Electroencephalogr Clin Neurophysiol 9: 673–90. doi:10.1016/0013-4694(57)90088-3. 
  5. ^ Rechtschaffen A, Kales A, editors. A Manual of Standardized Terminology, Techniques and Scoring System for Sleep Stages of Human Subjects. Washington: Public Health Service, US Government Printing Office; 1968.
  6. ^ Iber, C; Ancoli-Israel, S; Chesson, A; Quan, SF for the American Academy of Sleep Medicine (2007). The AASM Manual for the Scoring of Sleep and Associated Events: Rules, Terminology and Technical Specifications.. Westchester: American Academy of Sleep Medicine. 
  7. ^ Psychology World (1998). "Stages of Sleep" (PDF). http://web.mst.edu/~psyworld/general/sleepstages/sleepstages.pdf. Retrieved on 2008-06-15. "(includes illustrations of "sleep spindles" and "K-complexes")" 
  8. ^ Schulz, Hartmut (2008). "Rethinking sleep analysis. Comment on the AASM Manual for the Scoring of Sleep and Associated Events" (Full text). J Clin Sleep Med (American Academy of Sleep Medicine) 4 (2): 99–103. http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2335403. Retrieved on 2009-01-04. 
  9. ^ Swierzewski, Stanley J., M.D. (01 December 2000, reviewed 04 December 2007). "Sleep Stages. Overview, Waking, Non-REM, REM, Sleep Cycle, Factors, Age". Sleep Channel, Healthcommunities.com. http://www.sleepdisorderchannel.com/stages/. Retrieved on 2008-02-10. 
  10. ^ Wyatt, James K.; Ritz-De Cecco, Angela; Czeisler, Charles A.; Dijk, Derk-Jan (01 October 1999). "Circadian temperature and melatonin rhythms, sleep, and neurobehavioral function in humans living on a 20-h day". Am J Physiol 277 (4): R1152–R1163. Fulltext. PMID 10516257. http://ajpregu.physiology.org/cgi/content/full/277/4/R1152. Retrieved on 2007-11-25. 
  11. ^ Zisapel, N. (2007). "Sleep and sleep disturbances: biological basis and clinical implications." (Abstract). Cell Mol Life Sci 64 (10): 1174–86. doi:10.1007/s00018-007-6529-9. http://www.websciences.org/cftemplate/NAPS/archives/indiv.cfm?ID=20066335. Retrieved on 2009-01-05. 
  12. ^ a b de Benedictis, Tina, Ph.D.; Heather Larson, Gina Kemp, M.A., Suzanne Barston, Robert Segal, M.A. (2007). "Understanding Sleep: Sleep Needs, Cycles, and Stages". Helpguide.org. http://www.helpguide.org/life/sleeping.htm. Retrieved on 2008-01-25. 
  13. ^ Dijk, Derk-Jan; Steven W. Lockley (February 2002). "[http://jap.physiology.org/cgi/content/full/92/2/852 Functional Genomics of Sleep and Circadian Rhythm Invited Review: Integration of human sleep-wake regulation and circadian rhythmicity]". J Appl Physiol 92 (2): 852–62. doi:10.1152/japplphysiol.00924.2001. http://jap.physiology.org/cgi/content/full/92/2/852. Retrieved on 2009-04-04. "Consolidation of sleep for 8 h or more is only observed when sleep is initiated ~6-8 h before the temperature nadir.". 
  14. ^ Wyatt, James K.; Ritz-De Cecco, Angela; Czeisler, Charles A.; Dijk, Derk-Jan (01 October 1999). "Circadian temperature and melatonin rhythms, sleep, and neurobehavioral function in humans living on a 20-h day". Am J Physiol 277 (4): R1152–R1163. PMID 10516257. http://ajpregu.physiology.org/cgi/content/full/277/4/R1152. Retrieved on 2007-11-25. "... significant homeostatic and circadian modulation of sleep structure, with the highest sleep efficiency occurring in sleep episodes bracketing the melatonin maximum and core body temperature minimum". 
  15. ^ a b ""Let Sleep Work for You" provided by the National Sleep Foundation". http://www.sleepfoundation.org/site/c.huIXKjM0IxF/b.2421185/k.7198/Let_Sleep_Work_for_You.htm. 
  16. ^ Van Dongen HP, Maislin G, Mullington JM, Dinges DF. The cumulative cost of additional wakefulness: dose-response effects on neurobehavioral functions and sleep physiology from chronic sleep restriction and total sleep deprivation. Sleep. 2003 Mar 15;26(2):117–26.
  17. ^ Rhonda Rowland (2002-02-15). "Experts challenge study linking sleep, life span". http://archives.cnn.com/2002/HEALTH/02/14/sleep.study/index.html. Retrieved on 2007-04-22. 
  18. ^ Patel SR, Ayas NT, Malhotra MR et al. (2004) A prospective study of sleep duration and mortality risk in women Sleep 1;27(3):440-4
  19. ^ Patel SR, Malhotra A, Gottlieb DJ et al. (2006) Correlates of long sleep duration Sleep 29(7):881-889; cf. Irwin MR, Ziegler M (2005) Sleep deprivation potentiates activation of cardiovascular and catecholamine responses in abstinent alcoholics Hypertension 45(2):252-7
  20. ^ ""Researchers say lack of sleep doubles risk of death… but so can too much sleep"". http://www2.warwick.ac.uk/newsandevents/pressreleases/researchers_say_lack/. 
  21. ^ Jane E. Ferrie, Martin J. Shipley, Francesco P. Cappuccio, Eric Brunner, Michelle A. Miller, Meena Kumari, and Michael G. Marmot. "A prospective study of change in sleep duration; associations with mortality in the Whitehall II cohort." SLEEP Journal.
  22. ^ Siegel, Jerome M. (1999). "Sleep". Encarta Encyclopedia. Microsoft. http://www.npi.ucla.edu/sleepresearch/encarta/Article.htm. Retrieved on 2008-01-25. 
  23. ^ "Sleep Drive and Your Body Clock". National Sleep Foundation. Undated. http://www.sleepfoundation.org/site/c.huIXKjM0IxF/b.4809547/k.5094/Sleep_Drive_and_Your_Body_Clock.htm. Retrieved on 2009-04-03. "...this sleep phase delay can make it difficult to get the sleep teens need -- an average of 9 1/4 hours, but at least 8 1/2 hours." 
  24. ^ Gottselig, J. M.; Adam M; Rétey, J. V.; Khatami, R.; Achermann, P.; Landolt, H.P. (March 2006). "Random number generation during sleep deprivation: effects of caffeine on response maintenance and stereotypy" (Abstract). J Sleep Res 15 (1): 31–40. doi:10.1111/j.1365-2869.2006.00497.x. PMID 16490000. http://www.ncbi.nlm.nih.gov/pubmed/16490000?ordinalpos=3&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_DefaultReportPanel.Pubmed_RVDocSum. Retrieved on 2009-01-05. 
  25. ^ Iglowstein, Ivo; Oskar G. Jenni, MD, Luciano Molinari, PhD and Remo H. Largo, MD (February 2003). "Sleep Duration From Infancy to Adolescence: Reference Values and Generational Trends". Pediatrics 111 (2): 302–307. doi:10.1542/peds.111.2.302. PMID 12563055. "Thus, the shift in the evening bedtime across cohorts accounted for the substantial decrease in sleep duration in younger children between the 1970s and the 1990s. ... [A] more liberal parental attitude toward evening bedtime in the past decades is most likely responsible for the bedtime shift and for the decline of sleep duration...". 
  26. ^ Cirelli, Chiara; Giulio Tononi (August 26, 2008). "Is Sleep Essential?" (Essay). PLoS Biol (Public Library of Science) 6 (8): e216. doi:10.1371/journal.pbio.0060216. http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0060216&ct=1. Retrieved on 2009-04-21. "... it would seem that searching for a core function of sleep, particularly at the cellular level, remains a worthwhile exercise". 
  27. ^ Reference list is found on image page in Commons: Commons:File:Effects of sleep #References
  28. ^ Gumustekin, K.; Seven, B., Karabulut, N., Aktas, O., Gursan, N., Aslan, S., Keles, M., Varoglu, E., Dane S. (2004). "Effects of sleep deprivation, nicotine, and selenium on wound healing in rats" (Abstract). Int J Neurosci 2004;114(11): 1433-42. http://websciences.org/cftemplate/NAPS/archives/indiv.cfm?ID=20044648. 
  29. ^ Zager, A., Andersen, M.L., Ruiz, F.S., Antunes, I.B., & Tufik, S. (2007). Effects of acute and chronic sleep loss on immune modulation of rats [Electronic version]. Regulatory, Integrative and Comparative Physiology, 293, R504-R509.
  30. ^ Opp, Mark R. (January 2009). "Sleeping to fuel the immune system: mammalian sleep and resistance to parasites" (Full text, Creative Commons Attribution License). BMC Evolutionary Biology (BioMed Central Ltd.) 9 (8): 1471-2148. doi:10.1186/1471-2148-9-8. http://www.biomedcentral.com/1471-2148/9/8. Retrieved on 2009-06-28. 
  31. ^ Jenni, O. G., Molinari, L., Caflisch, J. A., & Largo, R. H. (2007). Sleep duration from ages 1 to 10 years: Variability and stability in comparison with growth [Electronic version]. Pediatrics, 120, e769-e776.
  32. ^ Van Cauter, E., Leproult, R., & Plat, L. (2000). Age-related changes in slow-wave sleep and REM sleep and relationship with growth hormone and cortisol levels in healthy men [Electronic version]. Journal of the American Medical Association, 284, 861-868.
  33. ^ Daan S, Barnes BM, Strijkstra AM (1991). "Warming up for sleep? Ground squirrels sleep during arousals from hibernation". Neurosci. Lett. 128 (2): 581. doi:10.1016/0304-3940(91)90276-Y. PMID 1945046. http://linkinghub.elsevier.com/retrieve/pii/0304-3940(91)90276-Y. 
  34. ^ Guidelines for the Care and Use of Mammals in Neuroscience and Behavioral Research. Institute for Laboratory Animal Research (ILAR), National Research Council. The National Academies Press. 2003. pp. pg 121. ISBN 978-0-309-08903-6. http://books.nap.edu/openbook.php?record_id=10732&page=121. "Sleep deprivation of over 7 days with the disk-over-water system results in the development of ulcerative skin lesions, hyperphagia, loss of body mass, hypothermia, and eventually septicemia and death in rats (Everson, 1995; Rechtschaffen et al., 1983)." 
  35. ^ Amanda Schaffer (Mai 27, 2007). "Why do we Sleep?". Slate.com. http://www.slate.com/id/2162475/entry/2162477/. Retrieved on 2008-08-23. 
  36. ^ Turner, T.H., Drummond, S.P.A., Salamat, J.S., & Brown, G.G. (2007). Effects of 42 hr sleep deprivation on component processes of verbal working memory [Electronic version]. Neuropsychology, 21, 787-795.
  37. ^ Born, J., Rasch, J., & Gais, S. (2006). Sleep to remember [Electronic version]. Neuroscientist, 12, 410.
  38. ^ Datta, S. (2000). Avoidance task training potentiates phasic pontine-wave density in the rat: A mechanism for sleep-dependent plasticity [Electronic version]. The Journal of Neuroscience, 20, 8607-8613.
  39. ^ Kudrimoti, H.S., Barnes, C.A., & McNaughton, B.L. (1999). Reactivation of hippocampal cell assemblies: Effects of behavioral state, experience, and EEG dynamics [Electronic version]. The Journal of Neuroscience, 19, 4090-4101.
  40. ^ "Nature, 10/05
  41. ^ Nanayakkara, Anushka. "Dream your dreams away". The Observer. http://www.sundayobserver.lk/2008/06/22/spe18.asp. Retrieved on 2008-07-09. 
  42. ^ Hobson, J.A., & McCarley, R. (1977). The brain as a dream state of sex: An activation-synthesis hypothesis of the dream process. American Journal of Psychiatry, 134, 1335–1348.
  43. ^ http://www.sleepdex.org/alcohol1.htm
  44. ^ http://alcoholism.about.com/cs/alerts/l/blnaa41.htm
  45. ^ Abarca C, Albrecht U, Spanagel R. Cocaine sensitization and reward are under the influence of circadian genes and rhythm. Department of Psychopharmacology, Central Institute of Mental Health, University of Heidelberg, J5, 68159 Mannheim, Germany.
  46. ^ Primary hypersomnia: Diagnostic Features
  47. ^ Little, Nan (2007-01-01). "What Causes Sleep Difficulty?". Insight Journal. http://www.anxiety-and-depression-solutions.com/wellness_concerns/sleep/sleep_problem_causes.php. Retrieved on 2008-01-25. 
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  50. ^ a b Carol M. Worthman and Melissa K. Melby. "6. Toward a comparative developmental ecology of human sleep" (PDF). A comparative developmental ecology. Emory University. 
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  52. ^ Faraco, Juliette (2000-08-01). "Re: Are there animals who don't sleep or that sleep very little?". MadSci Network: Zoology. http://www.madsci.org/posts/archives/2000-08/965504574.Zo.r.html. Retrieved on 2008-01-25. 
  53. ^ Insomnia Mania: Newborn Mammals Don't Sleep for a Month | LiveScience

External links


Wiktionary

Up to date as of January 15, 2010

Definition from Wiktionary, a free dictionary

Contents

English

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Wikipedia

Most common English words: ago « easily « condition « #686: sleep » ex » mere » agreement

Etymology 1

From Old English slǣp, from Germanic. Cognate with Dutch slaap, German Schlaf.

Pronunciation

Noun

Singular
sleep

Plural
countable and uncountable; plural sleeps

sleep (countable and uncountable; plural sleeps)

  1. (uncountable) The state of reduced consciousness during which a human or animal rests in a daily rhythm.
    I really need some sleep.
  2. (countable) (informal) An act or instance of sleeping.
    I’m just going to have a quick sleep.
  3. (uncountable) A substance found in the corner of the eyes after waking, whether real or a figurative objectification of sleep (in the sense of reduced consciousness).
    Wipe the sleep from your eyes.
Synonyms
Derived terms
Translations
See also

Category:Sleep

Etymology 2

From Old English slǣpan.

Pronunciation

Verb

Infinitive
to sleep

Third person singular
sleeps

Simple past
slept

Past participle
slept

Present participle
sleeping

to sleep (third-person singular simple present sleeps, present participle sleeping, simple past and past participle slept)

  1. (intransitive) To rest in a state of reduced consciousness.
    You should sleep 8 hours a day.
  2. (transitive, informal) To have sex with.
    The passionate lovers slept with each other every night.
  3. (transitive) To accommodate in beds.
    This caravan can sleep up to four people.
Derived terms
Troponyms
Translations

Anagrams

  • Anagrams of eelps
  • peels

Dutch

Noun

sleep m.

  1. train: the part of wedding gown that drags behind the bride

See also

Verb

sleep

  1. first person singular present tense of slepen
  2. singular imperative of slepen
  3. singular past tense of slijpen

Simple English

File:A child
A child sleeping
Simple English Wiktionary has the word meaning for:

Sleep is a state of rest, which happens in animals, including humans. Animals that sleep are in an unconscious state, or mostly so. Most of the muscles that animals can control on purpose are inactive.[1] Animals that are asleep do not react to stimuli as fast as those that are awake. They can wake up from sleep more easily than from hibernation or coma. All mammals and birds, and many reptiles, amphibians, and fish, sleep. In humans, other mammals, and most other animals that were studied, sleeping regularly is essential for survival.

The purpose of sleep is not fully known, and it is still studied today.[2] We do know for sure, though, that sleep is necessary to most animals for survival.[3] Not having enough sleep can even be used for torture.

A person sleeps when he or she is not awake. Usually, this is during the night. During the day, most people are awake. They work, go to school or university, or do other things. Many people sleep for a short time in the early afternoon for a quick rest. This is called a nap.

Contents

Word

The word "sleep" comes from the old Old Germanic verbs for sleep.[4] In Old and Middle High German it was called "SLAF". The original meaning of the word was "to slap", which was related to the word for "flabby" (not hard or firm).[5]

Many words related to "sleep" have very different meanings, though. For example, "sleep" was also used to mean death, so that "putting an animal to sleep" meant to kill the animal without pain.[4] "Sleep with someone" can also have a sexual meaning.[4]

What sleep is for

Generally, the reason for sleep is that the brain has work to do during sleep. The details are not fully understood, but it is important to get enough sleep for the body and the brain to be healthy and work properly. Usually animals (and people) sleep at periodic intervals, such as once a day. Certain animals send out signals to the others that they will soon go to sleep. Yawning is such a signal.

Both humans and many animals sleep about once a day. Some animals, such as cats, sleep many times a day for short periods.

When people sleep they often have dreams. Probably some animals do, too.

Not only people sleep, all mammals and birds, and most fish, reptiles and other animals do, too.

Different categories of sleep

In mammals and birds, sleep can be divided into two categories. In one of them, the eyes move rapidly. It is called REM-sleep (from rapid eye movement). Most dreams take place in this phase. REM-sleep occurs normally at intervals throughout the night, and the periods of REM-sleep increase in length in the second half of the night. REM-sleep was first discovered in 1952-53.

The other category, where this movement of the eyes does not happen, is called NREM-sleep (Non-REM sleep). Usually, dreams do not occur during this time. There are three or four stages of NREM-sleep. Stage I is just barely sleeping, or dozing. Stage II is also light sleep. Normally, in adult humans, about half of the time spent asleep is spent in light sleep. Stages III and IV are called deep sleep. Deep sleep is necessary for growth and healing. It can be quite difficult to awaken someone who is in stage III or stage IV sleep. Sometimes stages III and IV are combined and called stage III.

Adult humans normally sleep in cycles of 90 to 110 minutes each. The night's sleep can be 4 or 5 of these cycles. Each cycle includes, in this order: stage I, stage II, stage III (IV), stage II and REM.

Getting enough sleep

The timing of sleep and the amount of it are both important. Both are different for different people. Some adults sleep best from 22:00 to 05:00 or 06:00 or 07:00. Some sleep best from midnight to seven or eight. These variations are normal.

How much sleep is enough also depends on age. Children need more sleep than adults. Newborn babies sleep about 18 hours per day. Small babies sleep many times a day; human babies do not develop circadian rhythms before they are 3 - 4 months old. At the age of 1 year, they sleep for about 14 hours.

A nine-year-old should sleep about 9-10 hours per day and teenagers, too, need that much sleep. Adults who sleep less than about 8 hours a day perform worse than those who sleep that long. [6][7]

Sleeping problems

People may have trouble going to sleep, staying asleep or getting enough sleep. This usually means that they are too sleepy in the daytime.

There are many things that influence sleep. Also some substances, called stimulants - coffee is such an example - can cause poor sleep. When people have just eaten something, the body is busy digesting. This can cause poor sleep, too. Worrying and stress can cause poor sleep.

There are many diseases that cause poor sleep. Fever can lead to bad dreams. Poor sleep can be a side effect of some medications.

Sleep disorders directly influence how a person sleeps. Examples of sleep disorders are narcolepsy, sleep apnea and circadian rhythm sleep disorders.

Sleep specialists - doctors specialised in sleeping problems - often suggest better sleep hygiene to people with sleeping problems. Sleep hygiene means things people can try, such as

  • avoid extreme emotion in the hours before sleep
  • trying to get up at the same time every day,
  • sleeping in a cool, quiet and very dark place,
  • avoiding bright light the last hour before bedtime,
  • avoiding a big meal just before bedtime, and
  • getting enough exercise every day.

References

  1. Macmillan Dictionary for Students Macmillan, Pan Ltd. (1981), page 936. Retrieved 2009-10-1.
  2. Bingham, Roger; Terrence Sejnowski, Jerry Siegel, Mark Eric Dyken, Charles Czeisler, Paul Shaw, Ralph Greenspan, Satchin Panda, Philip Low, Robert Stickgold, Sara Mednick, Allan Pack, Luis de Lecea, David Dinges, Dan Kripke, Giulio Tononi (February 2007). "Waking Up To Sleep" (Several conference videos). The Science Network. http://thesciencenetwork.org/programs/waking-up-to-sleep. Retrieved 2008-01-25. 
  3. Guidelines for the Care and Use of Mammals in Neuroscience and Behavioral Research, Institute for Laboratory Animal Research (ILAR), National Research Council, S. pg 121, The National Academies Press 2003, ISBN 978-0-309-08903-6
  4. 4.0 4.1 4.2 "Online Etymology Dictionary". etymonline.com. http://www.etymonline.com/index.php?search=sleep&searchmode=none. Retrieved 29 September 2010. 
  5. Alexander Borbély: Das Geheimnis des Schlafs. Deutsche Verlags-Anstalt, Stuttgart 1984. ISBN 3-421-02734-X
  6. ""Let Sleep Work for You" provided by the National Sleep Foundation". http://www.sleepfoundation.org/site/c.huIXKjM0IxF/b.2421185/k.7198/Let_Sleep_Work_for_You.htm. 
  7. Rhonda Rowland (2002-02-15). "Experts challenge study linking sleep, life span". http://archives.cnn.com/2002/HEALTH/02/14/sleep.study/index.html. Retrieved 2007-04-22. 

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