Sleep

For other uses, see Sleep (disambiguation).

Sleep is the state of natural rest observed in most mammals, birds, fish, as well as invertebrates such as the fruitfly Drosophila. It is characterized by a reduction in voluntary body movement, decreased reaction to external stimuli, an increased rate of anabolism (the synthesis of cell structures), and a decreased rate of catabolism (the breakdown of cell structures). In humans, mammals and many other animals which have been studied, such as fish, birds, mice, ants and fruitflies, regular sleep is necessary for survival. The capability for arousal from sleep is a protective mechanism and also necessary for health and survival.

Arousal

Main article: Arousal

The capability for arousal from sleep provides an organism with the energy, strength, and opportunity to become mobile and to direct its sensory organs to locate and ingest food, to procreate, and avoid predation or other potentially dangerous situations. The energy and strength of a mobile organism, and its sensory organs serve to protect an animal in its search for nutrition and in its ability to ingest it, thus ensuring its survival. These same factors also make possible its attempts to procreate, thus insuring its success as an organism. The balance of sleep and wakefulness is essential to the survival and the success of every animal that sleeps.

Sleep physiology

Currently, scientists divide sleep into two broad types: REM (Rapid Eye Movement) and NREM (non-REM) sleep. They are very different from each other, so much so that some scientists consider them 2 of the 3 states of 'being': awake, non-REM sleep and REM sleep.

REM sleep is thought of as 'an active brain in an inactive body', and is the stage where most dreaming occurs. REM sleep is characterized by an EEG that is 'low voltage, mixed frequency', similar in appearance to the awake EEG. During REM sleep there is loss of skeletal muscle tone, and an active sympathetic nervous system. When penile erections occur during sleep they usually occur in REM sleep.

By contrast, non-REM sleep is 'an inactive brain in an active body'. There is relatively little dreaming. Non-REM encompasses 4 stages (1-4), with stages 1 and 2 'light sleep' and stages 3 and 4 'deep sleep'. They are differentiated solely by the EEG (in contrast to REM sleep, which requires relative absence of muscle tone and rapid eye movements to properly characterize). There are often limb movements in non-REM sleep; the parasomnia sleep walking occurs in non-REM.

Stage 4 Sleep. EEG highlighted by red box.

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

• Non-REM accounts for 75–80% of total sleep time in normal human adults, and consists of four stages:
  • Stage 1, with near-disappearance of the alpha waves seen in awake states, and appearance for the first time of theta waves. (Alpha waves have a frequency of 8-13 Hz (cycles/second). Theta waves have a frequency of 4-7 Hz.) Stage 1 is sometimes referred to as somnolence, or "drowsy sleep". It appears at sleep onset (as it is mostly a transition state into Stage 2) and is associated with the sudden twitches or hypnic jerks many people experience when falling asleep. While these are normal and of no concern, the hypnagogic hallucinations which some people may experience at this stage can be more troublesome. During this period, the subject loses some muscle tone, and conscious awareness of the external environment: Stage 1 can be thought of as a gateway state between wake and sleep.
  • Stage 2, with "sleep spindles" (12–16 Hz) and "K-complexes." The EMG lowers, and conscious awareness of the external environment disappears. This occupies 45–55% of total sleep.
  • Stage 3, with delta waves, also called delta rhythms (.5–4 Hz), is considered part of slow-wave sleep (SWS) and functions primarily as a transition into stage four. Overall it occupies 3–8% of total sleep time.
  • Stage 4 is true delta sleep. It predominates the first third of the night and accounts for 10–15% of total sleep time. This is often described as the deepest stage of sleep; it is exceedingly difficult to wake a subject in this state. This is the stage in which night terrors, bed wetting, sleepwalking, and sleep-talking occur.
• REM sleep 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. Also known as Stage 5 sleep.
• Active Sleep is a phase of sleep in neonates that appears similar to Rapid Eye Movement (REM) sleep in adults. While it depends on age, neonatal sleep is sometimes scored as Active Sleep, Quiet Sleep, and Wake. This is less specific than the classification of adult's sleep, and is often based on behavioral criteria due to the technical difficulties arising from recording EEG from the neonate.

Scientists are divided on the precise relation between Active Sleep and REM sleep. Some suggest that they are similar, while others say it is an entirely different state, which represents aspects of the developing CNS that are not present in a mature brain, and that certain aspects of REM are not present in Active Sleep due to the immaturity of the CNS in the neonate.

Sleep proceeds in cycles of NREM and REM phases. In humans, the cycle of REM and NREM is approximately 90 minutes. Each stage 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.

Each sleep stage is not necessarily uniform. Within a given stage, a cyclical alternating pattern may be observed.

Both REM sleep and NREM sleep stages 3 and 4 are homeostatically driven; that is, selective deprivation of each of these states subsequently causes a rebound in their appearance once the person is allowed to sleep. This finding leads to the ubiquitous assumption that both are essential in the sleep process and its many functions. REM sleep may also be driven by a circadian oscillator, as studies have shown that REM is temporally coupled with the circadian rhythm of temperature.

Regulation of sleep

The cycle of sleep and wakefulness is regulated by the brain stem, thalamus, external stimuli, and various hormones produced by the hypothalamus. Some neurohormones and neurotransmitters are highly correlated with sleep and wake states. For example, melatonin levels are highest during the night, and this hormone appears to promote sleep. Adenosine, a nucleotide involved in generating energy for biochemical processes, gradually accumulates in the human brain during wakefulness though decreases during sleep. Researchers believe that its accumulation during the day encourages sleep. The stimulant properties of caffeine are attributed to its negating the effects of adenosine. However 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.

Thus, three processes, each influenced by hormonal, neurological, and environmental factors, underlie sleep regulation:

• A homeostatic process determined by prior sleep and wakefulness, determining "sleep need."
• A circadian process determining periods of high and low sleep propensity, and high and low rapid eye movement (REM) sleep propensity.
• An ultradian process.

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

Theories regarding the function of sleep

Given sleep's heterogeneous nature, no single theory predominates, as it is difficult to describe one single "function" of sleep.

• Restorative theories of sleep describe sleep as 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 changes in immune function occur. In some studies sleep deprivation has led to decrements in immune function and, under extreme, extended sleep deprivation regimes, altered metabolism. However, 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.

• 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). Sleep might restore neurons and increase production of brain proteins and certain hormones. Wakefulness may perhaps be viewed as a cyclical, temporary, hyperactive catabolic state during which the organism acquires nourishment and procreates. Also, during sleep, an organism is vulnerable; when awake it may perceive and avoid threats. Asking the question "Why do we awaken?" instead of "Why do we sleep?" yields a different perspective toward understanding how sleep and its stages contribute to a healthy organism.

• 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).

• One process commonly believed to be highly dependent on sleep is memory. REM sleep appears to help with the consolidation of spatial and procedural memory, while slow-wave sleep helps with the consolidation of declarative memories. When experimental subjects are given academic material to learn, 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 saved first in a temporary memory store, and then encoded and transferred into long-term memory during sleep. (Zhang, 2004). Note that despite an abundance of positive findings in support of these ideas, many sleep scientists do not believe that sleep's primary function is related to memory. These scientists 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--which is the mechanism underlying remembering and forgetting (Benington and Frank, 2003).

• One view, "Preservation and Protection", is that sleep serves an adaptive function. It protects the individual during that portion of the 24-hour day in which being awake, and hence roaming around, would place the individual at greatest risk. Organisms don't 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). This theory, however, is not universally accepted. For example, if true, there would be no reason for the brain to disengage from the external environment as it does during normal sleep. A more advantageous adaptation would be for animals to seclude themselves but maintain quiet wakefulness to avoid predation. Nevertheless, it must be said that animals who are preyed upon usually disengage from the external environment to a lesser degree. Sleep is not simply a passive consequence of removing the animal from the environment, but rather is itself a "drive": animals will 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 most likely reflect different and unknown functions.

These several theories are not mutually exclusive; each may contain truths that will be validated in the future. Despite decades of intense research, scientists still have only clues to sleep function. With the recent demonstration that sleep is phylogenetically ancient (Shaw et al Science 2000, Hendricks et al Neuron 2000), the focus for understanding the purpose of sleep shifts from humans and other mammals to simple animals that predated the emergence of arthropoda and chordata phyla. Therefore, some of the sleep features that are unique to mammals (e.g. REM sleep and thermoregulation) are unlikely to have played a role in the evolution of a sleep-like state in the primordial metazoan. An examination of the nature of sleep and of wakefulness thus turns its focus to the study of the roles that proteins and enzymes play in basic metabolism.

Longest Record Without Sleep

Depending on how one defines sleep, there are several persons who can claim the record for having gone the longest without sleep. Aside from mere curiosity, these record may prove important to various sleep theories.

1. Thai Ngoc, a Sixty-four-year-old Vietnamnese (at 2006), have stayed awake for 33 years or 11,700 nights, according to Vietnamese news organization Thanh Nien. [1] He suffers from no apparent ill effect (other than the fact that he cannot sleep). Ngoc is mentally sound and is able to carry 100kg of pig feed down a 4km road. He acquired the ability to go without sleep after a bout of fever in 1973.

2. As far as "normal" persons go, Randy Gardner holds the world record for intentionally having gone the longest without sleep. In 1965, Gardner, then 17, stayed awake for 264 hours (about 11 days) for a high school science project. [2]He Gardner experienced significant deficits in concentration, motivation, perception and other higher mental processes during his sleep deprivation. However, he recovered normal cognitive functions after a full night's sleep.

3. People born with the rare genetic disorder Morvan’s fibrillary chorea or Morvan’s syndrome can go without sleep for several month 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. [3]

(In recent investigations, Morvan’s Syndrome has been attributed to serum antibodies directed against specific potassium (K+) channels in cell and nerve membranes. )

Optimal sleep amount

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, overall health, as well as reducing the risk of accidents. A widely publicized 2003 study performed at University of Pennsylvania School of Medicine demonstrated that cognitive performance declines with fewer than eight hours of sleep. It is also proven that one does not have to be sleeping for adequete growth.

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 correlation does not imply causation (e.g. such correlation can be explained from the fact that older people tend to sleep less).

Dreaming

Main article: Dream

Dreaming involves an involuntary conjuring up of images in a sequence in which the sleeper/dreamer is usually more a 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 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.

Another hypothesis is that dreaming allows an animal to play out scenarios that may help the animal avoid dangers when awake. For example, a rabbit might dream about being cornered by a fox and may play out different scenarios that might increase its chances of survival should it come across a fox in reality.

James Allan Hobson and Robert McCarley's activation synthesis theory proposes that dreams are caused by random firings 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.

Most recent studies of dreams functionality in University of Helsinki by professor Pekka Sutola suggest that non-REM sleep is rapid processing of experiences gathered by mammals during the day which are then applied to deep-sleep phase for postprocessing. This is also an explanation as to why epileptics entering REM-like stage during serious relapses can't remember much from the few hours preceeding the relapse. Complete function is still unknown.

Anthropology of sleep

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. For example, people in these cultures might go to sleep far more quickly after the sun sets, but then wake up several times throughout the night, sometimes staying awake for several hours. The boundaries between sleeping and waking are blurred in these societies. Some observers believe that sleep in these societies is most often split into two main periods, the first characterised primarily by "slow sleep" and the second by REM sleep. This is called segmented sleep, which 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 cultures have a siesta, in which people sleep for a period in the afternoon. In many nomadic or hunter-gatherer societies people will sleep off and on throughout the day or night depending on what is happening.

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

In some cultures people generally sleep with at least one other person, often with many, or with other types of animals. In other cultures 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, with their children, with 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.

People sleep in a variety of locations. Some sleep directly on the ground, others on a 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, housing type, and the incidence of pests.

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 (changing clothes in the morning), and some sleep naked.

Sleep in nonhumans

Sleeping Japanese macaques

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 period of 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.

Sleep and breathing

Main article: Sleep and breathing

Breathing patterns change significantly from wakefulness at sleep onset and during different sleep stages. Pathologic breathing during sleep results in increased morbidity and mortality.

See also

Sleep physiology	Circadian rhythm Myoclonic twitch Seasonal affective disorder Sleep inertia Sleep paralysis polysomnogram
Patterns and disruptions	Hibernation Jet lag Polyphasic sleep Segmented sleep Unihemispheric slow-wave sleep Sleep deprivation Sleep debt
Practices and rituals	Lucid Dreaming Co-sleeping Hypnosis Meditation Yoga Nidra Sleep hygiene

• Alarm clock
• Insomnia
• Hypersomnia
• Lullaby
• Narcolepsy
• Sudden infant death syndrome
• Dream dictionary

References

• 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
• World Sleep Foundation
• National Center on Sleep Disorders Research
• "It's the Little Things: Daily Routines". The Whole Child - For Early Care Providers. Public Broadcasting Service (PBS).