Sleep

For other uses, see Sleep (disambiguation).

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

In humans, other mammals, and many other animals that have been studied — such as fish, birds, ants, and fruit-flies — regular sleep is necessary for survival. The capability for arousal from sleep is a protective mechanism and also necessary for health and survival.

Characteristics

Sleep is generally characterized by a reduction in voluntary body movement, temporary blindness, decreased reaction to external stimuli, loss of consciousness, a 71% reduction in audio receptivity, an increased rate of anabolism (the synthesis of cell structures), and a decreased rate of catabolism (the breakdown of cell structures).

Physiology

Stages

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 110 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 fulfil its physiological functions.

In REM, the brain is active and the body inactive, and this is when most dreaming occurs. REM sleep is characterized by an electroencephalography (EEG) that has low voltage and mixed frequency, similar in appearance to the wakeful EEG. During REM sleep there is loss of skeletal muscle tone, and an active sympathetic nervous system.

In NREM sleep, the body is active and the brain 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.

Rechtschaffen and Kales originally outlined the criteria for staging sleep in 1969. 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.

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.

NREM

NREM accounts for 75–80% of total sleep time in normal human adults, and 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. Other people may also experience hypnagogic hallucinations during this stage, which can be more troublesome. 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. Overall it occupies 3 to 8% of total sleep time. 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 characteristic, such as delta-waves, are more pronounced.

REM

REM sleep (Stage R, or Stage 5) is popularly associated with dreaming, especially bizarre, visual, and seemingly random dreams; however, dreams can also occur during sleep onset (hypnagogia) and during all other stages of sleep. REM sleep is predominant in the final third of a sleep period; its timing is linked to circadian rhythm and body temperature. The EEG in this period is aroused and looks similar to stage 1, and sometimes includes beta waves.

• Active Sleep is a phase of sleep in newborns that appears similar to REM sleep in adults. Neonatal sleep depends on age and is sometimes divided into Active Sleep, Quiet Sleep, and Wake. This is less specific than the classification of an adult's sleep, and is often based on behavioral criteria because it is technically difficult to record an EEG from a newborn. Some scientists suggest that Active Sleep is similar to REM sleep. Others say it is an entirely different state, and point to differences between a developing central nervous system and a mature one.

Regulation

The cycle of sleep and wakefulness is regulated by the brain stem, thalamus, external stimuli, and various hormones produced by the hypothalamus. Three processes, influenced by hormonal, neurological, and environmental factors, underlie sleep regulation:

• A homeostatic process determined by prior sleep and wakefulness, determines "sleep need."
• A circadian process determines periods of high and low sleep propensity, and high and low REM sleep propensity.
• An ultradian process

The interrelationships and relative importance of each process and system remain uncertain.

Some neurohormones and neurotransmitters are highly correlated with sleep and wakeful states. For example, melatonin levels are highest during the night, and this hormone appears to promote sleep. Adenosine, a nucleoside involved in generating energy for biochemical processes, gradually accumulates in the human brain during wakefulness, and decreases during sleep. Researchers believe that its accumulation encourages sleep. The stimulant properties of caffeine are attributed to its negating the effects of adenosine. But the role of adenosine is far from proven, as mice lacking adenosine receptors display normal sleep patterns and normal responses to sleep deprivation.

The suprachiasmatic nucleus (SCN) of the hypothalamus plays an important role and also generates its own rhythm in isolation. In the presence of light it sends messages to the pineal gland that instruct it to cease secreting melatonin.

Breathing

Main article: Sleep and breathing

Breathing patterns change significantly from wakefulness at sleep onset and during different sleep stages.

Functions

Despite decades of intense research, scientists still have only clues about sleep function. Because sleep is heterogeneous, there are various theories none of which predominates.

Restoration

Sleep may be a dynamic time of healing and growth for organisms. For example, during stages 3 and 4, or slow-wave sleep, growth hormone levels increase, and immune function changes. In some studies, sleep deprivation led to decrements in immune function, and extreme, extended deprivation to altered metabolism. But sleep deprivation has not been conclusively shown to significantly impact organ, muscular, cardiac or other somatic function in ways that suggest that any of these systems are primarily influenced by sleep.

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

Many scientists believe that memory depends on sleep. REM sleep appears to help the consolidation of spatial and procedural memory, while slow-wave sleep helps with the consolidation of declarative memories. When experimental subjects are asked to memorize academic material, especially if it involves organized, systematic thought, their retention is markedly increased after a night's sleep. On the other hand, the effectiveness of mere rote memorization is similar with or without an intervening period of sleep. Some memory theorists argue that saving memory directly into long-term memory is a slow and error-prone process, and propose that cerebral input is first saved in a temporary memory store, and then encoded and transferred into long-term memory during sleep (Zhang, 2004). But although many findings support these ideas, many sleep scientists do not believe that sleep's primary function is related to memory. They point out that many of the studies cited by proponents of this theory are contradictory or confounded by the side-effects caused by the experimental manipulations. A more salient issue is that only a handful of studies have shown that sleep actually influences brain plasticity, the mechanism underlying remembering and forgetting (Benington and Frank, 2003).

Preservation

One view, "Preservation and Protection", is 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

Adult

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.

Children

Children need a relatively larger amount of sleep 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

Longest period without sleep

Depending on how one defines sleep, there are several persons 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 (K+) channels in cell and nerve membranes.

Issues affecting sleep

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)
• Poor body positioning
• Illness

A study by researchers at the University of Pennsylvania has confirmed that the more one works, the less they sleep 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.

Partner

In some cultures people generally sleep with at least one other person, often with many, or with animals. In other cultures people rarely sleep with anyone but an 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, with their children, with children of a certain age, children of specific sex, peers of a certain sex, friends, peers of equal social rank, or with no one at all.

Location

People sleep in a variety of locations. Some sleep directly on the ground, others on an animal skin, piece of cloth or mattress, others sleep on platforms or beds. Some sleep with blankets, some with pillows, some with simple head rests, some with no head support. These choices are shaped by a variety of factors such as culture, climate, protection from predators and housing type.

Clothing

Clothing worn for sleep varies across individuals and cultures. Some people wear pajamas, some a nightshirt, some regular underwear, some the same clothes they wore during the day (some of them changing clothes in the morning), and some sleep naked.

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

• Circadian rhythm
• Myoclonic twitch
• Seasonal affective disorder
• Sleep inertia
• Sleep paralysis
• Polysomnogram

Patterns and disruptions

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

Practices and rituals

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

Other

• Dream world (plot device)
• Insomnia
• Hypersomnia
• Narcolepsy
• Sudden infant death syndrome

• Lullaby
• Alarm clock
• Dream interpretation

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