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, regular sleep is necessary for survival. 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

The National Sleep Foundation maintains that 8 hours of sleep is optimal, claiming that it brings improved performance in tests, reduced risk of accidents, and a better immune system.[citation needed] A widely publicized 2003 study[1] performed at University of Pennsylvania School of Medicine demonstrated that cognitive performance declines with less than eight hours of sleep.

An American study found that people who live the longest sleep for 4 to 7 hours each night[2]. However, this study cannot be used to determine optimal sleep amount because the study did not find cause, only correlation - and correlation does not imply causation.


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.[citation needed] When children are getting enough rest, they are more likely to:

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

The [1] (US) 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:

Helping children deal with "night terrors": Some children, especially between the age of 2-6 years, suffer from "night terrors". In practical terms, this means they awake from sleep screaming, and can take a considerable time to pacify. Very often these episodes will occur every night, and at about the same time. A proven practical solution is to fully wake the child around 30 minutes before the event is most likely to occur, and then allow them to sleep as normal. A week of this routine has been shown to help the majority of children that have this problem.


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; whereas non-rapid eye movement sleep is characterized by parasympathetic nervous system activity (Legramante & Galante, 2005).

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.

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:

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.

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. Dreaming is stimulated by the pons and occurs during the REM phase of sleep.[citation needed]

People have proposed many hypotheses about functions of dreaming. 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.[citation needed] 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.[citation needed]

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. However, by studying brainwaves it has been discovered that reptiles do not dream.

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.[3] 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.[citation needed]

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.[citation needed]

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.[citation needed]

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, 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 nude.


Sleep in nonhumans

Sleeping Japanese macaques
Sleeping Japanese macaques

Cattle, horses, and sheep can sleep while standing, though they do not experience REM sleep in this position, for REM sleep causes partial muscle paralysis. For REM sleep to take place, the animals must lie down.[citation needed] Some birds 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.[citation needed] While dreaming, dogs may make a quiet barking sound while both cats and dogs may make running motions with their legs.[citation needed]


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
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




  1. 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.
  2. Experts challenge study linking sleep, life span.
  3. Carol M. Worthman and Melissa K. Melby. “6. Toward a Comparative Developmental Ecology of Human Sleep”, A Comparative Developmental Ecology (PDF), Emory University.

External links

Retrieved from "http://localhost../../../art/a/n/s.html"

This text comes from Wikipedia the free encyclopedia. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. For a complete list of contributors for a given article, visit the corresponding entry on the English Wikipedia and click on "History" . For more details about the license of an image, visit the corresponding entry on the English Wikipedia and click on the picture.