2007 Schools Wikipedia Selection. Related subjects: Everyday life

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 and fruitflies, sleep is necessary for life. The capability for arousal from sleep is a protective mechanism and also necessary for health and survival. Sleep is not synonymous with unconsciousness.

For many, a bed, blankets, pillows, and pajamas are all traditionally associated with sleep.
For many, a bed, blankets, pillows, and pajamas are all traditionally associated with sleep.

Optimal sleep amount

An American study linked life spans with sleep amount and found that those who sleep 4 to 7 hours live longer lives, with those sleeping less than 4 or more than 9 hours having about a 30% increased chance of early death, similar to moderate obesity.. The National Sleep Foundation however maintains that 8 hours of sleep is optimal, claiming improved performance in tests, reduced risk of accidents and a better immune system. However, this does not apply to children and adolescents, particularly young children who require as much as 13 hours a night.

Sleep physiology

The cycle between sleep and wakefulness involves different stages of sleep. Currently, scientists divide sleep into two general types: REM ( Rapid Eye Movement) and NREM ( non-REM). REM sleep is characterized by desynchronization of the electroencephalograph (EEG), loss of skeletal muscle tone, and sympathetic nervous system activity; where as non-rapid eye movement sleep is characterized by parasympathetic nervous system activity (Legramante & Galante, 2005).

Stages of sleep

  • Non-REM accounts for 75–80% of total sleep time:
    • Stage 1, with near-disappearance of the alpha waves seen in awake states, and appearance for the first time of theta waves. The stage 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, and sleepwalking occur.
Stage 4 Sleep. EEG highlighted by red box.
Stage 4 Sleep. EEG highlighted by red box.
REM Sleep. EEG highlighted by red box. Eye movements highlighted by red line.
REM Sleep. EEG highlighted by red box. Eye movements highlighted by red line.
  • REM sleep is popularly associated with dreaming, especially bizarre, visual, and seemingly random dreams; however, dreams can also occur during sleep onset (hypnogogia) and during all stages of Non-REM 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.

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

  • 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, short periods of sleep deprivation have 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 behavioural problems, permanent sleep disruption, decreased brain mass (Mirmiran et al. 1983), and an abnormal amount of neuronal cell death (Morrissey, Duntley & Anch, 2004). Given sleep's heterogeneous nature, however, no single theory predominates, as it is difficult to describe one single "function" of sleep.
  • 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).
    • 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. 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 premordial 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.


Dreaming involves an involuntary conjuring up of images in a sequence in which the sleeper/dreamer is usually more a participant than an observer. Most scientists agree that dreaming is stimulated by the pons and occurs during the REM phase of sleep.

Many functions have been hypothesized for dreaming. Freud postulated that dreams are the symbolic expression of frustrated desires that had been relegated to the subconscious, and used dream interpretation in the form of psychoanalysis he pioneered. Scientists today have generally become more skeptical about details of 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.

Hobson and 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.

Anthropology of sleep

Recent research suggests that sleep patterns vary significantly across cultures. The most striking differences are between societies that have plentiful sources of artificial light and ones that do not. The primary difference appears to be that pre-light cultures have more broken up sleep patterns. For example, people might go to sleep far more quickly after the sun sets, but would then wake up several times throughout the night, punctuating their sleep with periods of wakefulness, perhaps lasting 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 in pre-industrial societies.

Some societies display a fragmented sleep pattern in which people sleep at all times of the day and night for shorter periods. For example, many Mediterranean societies 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.

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

In some societies people generally sleep with at least one other 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, 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 or blanket, 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 climate, protection from predators, housing type, technology, and the incidence of pests.

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

Sleep in animals

Sleeping Japanese macaques.
Sleeping Japanese macaques.

Cattle, horses, and sheep are unique in that they can sleep while standing, though none may experience REM sleep in this position, for REM sleep causes partial muscle paralysis. For REM sleep to take place, the animals must lie down. Conversely, birds may have periods of REM sleep while perched. Some breeds of dogs usually sleep throughout the day like cats, while other breeds have only one daily sleep session. While dreaming, dogs may make a quiet barking sound while both cats and dogs may make running motions with their legs.

Sleeping routines for young children

Children ages three to five, on average, should be sleeping 11-13 hours a night to promote optimal development and growth. When children are getting enough rest, they are more likely to:

  • Be more alert
  • Have a better memory
  • Show increased performance on tasks
  • Be less prone to behavioural problems
  • Have more mood stability

Children ages 6 to 12 need 8-10 hours of sleep.

The National Sleep Foundation recommends the main tip for promoting healthy sleeping habits in children is to follow a nightly routine. Achieving a nightly schedule may be difficult at first, especially if a child is not accustomed to a routine.

A bedtime ritual that children are familiar with will:

  • Make it easier to relax
  • Help them fall asleep on their own
  • Encourage them to sleep through the night

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