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       For other uses, see Sleep (disambiguation).

A bed, blankets, pillows, and pajamas are all traditionally associated with sleep

Sleep is the regular state of natural rest observed in all mammals, birds, and

fish. Sleep is not actually "unconsciousness," but rather, it is a natural state of

rest 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). Therefore, since consciousness is literally the awareness of the

surroundings, being asleep is just an altered state of consciousness, as

opposed to being unconscious. It is heavily influenced by circadian rhythms

and by hormonal and environmental factors. Sleep performs a restorative

function for the brain and body as evidenced by the myriad symptoms of

metabolic dysfunction that result when animals are deprived of sleep.
The function of sleep in health and in disease is being increasingly studied in

specialized sleep laboratories throughout the world. Not only insomnia but

more recently elucidated sleep disorders such as sleep apnea and narcolepsy

are evaluated in such facilities. The increasing prevalence of sleep disorders

is likely to be a function both of more sophisticated diagnostic tests and the

disruption of the normal day-night cycle in modern societies. [citation needed]

Sleep physiology


Before advances in the fields of neurology, neuroscience, electronics and

genetics were made, scientists studied the behavioral characteristics of sleep,

such as its pattern, depth, and varying frequency. In more recent times, the

electrical impulses generated by the brain are recorded using a device called

an electroencephalograph (EEG), and individual genes relating to

sleep-related brain function, such as the circadian rhythm, have been isolated.

Molecular biology, medical science and epidemiology all play an important

role in modern studies of sleep.

Sleep is often defined using specific criteria relating to EEG data. All

mammals and birds fulfill the criteria for sleep based on EEG recordings. In
animals where EEG data is not readily available, or their small size precludes

recording an EEG, behavioral and gene specific data are utilized for sleep



Sleep regulation

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 nucleoside involved in

generating energy for biochemical processes, gradually accumulates in the

human brain during wakefulness but 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.

The suprachiasmatic nucleus (SCN) of the hypothalamus plays an important

role in the regulation of circadian rhythms. The SCN is influenced by external

light 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

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.


Stages of sleep
Stage 1 Sleep. EEG highlighted by red box.

Stage 2 Sleep. EEG highlighted by red box. Sleep spindles highlighted by red line.

Stage 4 Sleep. EEG highlighted by red box.

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

Studies of human sleep have established five well-defined stages, according

to electroencephalographic (EEG) recordings and polysomnography:

      Non-REM sleep (NREM), accounts for 75-80% of total sleep time:
             o   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 can be
                 associated with so-called hypnagogic hallucinations. In 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.

             o   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.

             o   Stage 3, with delta waves, also called delta rhythms (1–2 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.

             o   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, bedwetting, and
                 sleepwalking occur.

      Rapid eye movement sleep (REM), associated with dreaming, especially
       bizarre, visual, and seemingly random dreams. REM sleep is predominant in
       the final third of a sleep period; its timing 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 below). This can result in a

sleep that exhibits loss of consciousness but does not fulfill its physiological

Each sleep stage is not necessarily uniform. Within a given stage, a cyclical

alternating pattern may be observed.


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. The many illnesses associated with sleep deprivation testify to its

restorative function.

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). Given

sleep's heterogeneous nature, however, no single theory predominates, as it

is difficult to describe one single "function" of sleep.
One process known 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. Mere rote memorization is retained similarly

well with or without an intervening period of sleep.

Non-REM sleep is an anabolic state marked by physiological processes of

growth and rejuvenation of the organism's immune, nervous, muscular, and

skeletal systems. Sleep also restores neurons and increases 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.

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 maximizes their safety, given

their physical capacities and their habitats. (Allison & Cicchetti, 1976; Webb,


Another view is that the function of sleep is for memory processing. This

theory argues that saving memory directly into the long-term memory is a

slow and error prone process, and, thus, proposes that the memory formed

during waking time is not saved directly into the long-term memory; instead it

is saved into a temporary memory store first. The function of sleep is to

process, encode and transfer the data from the temporary memory store to

the long-term memory store. (Zhang, 2004).

These theories are not mutually exclusive.


         Main article: Dream

The demonstrably necessary phenomenon of dreaming would suffice to prove

the importance of sleep to humans, and perhaps to other animals as well.
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 theory 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.


Anthropology of sleep

Recent research suggests that sleep patterns vary significantly across

cultures. [1] 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.


Sleep deprivation
         Main article: Sleep deprivation
A common misconception is that everyone needs eight hours of sleep. The

amount of sleep actually needed is individually and biologically determined,

and is different for each person. Some can do with six hours of sleep; others

need nine. However, as a general rule, eight hours is recommended. Sleep

experts state that one cannot "store" sleep by sleeping more on the weekends

in preparation for the normal work week.[2]

Another commonly held view is that the amount of sleep one requires

decreases as one ages, but this is not necessarily the case. The ability to

sleep, rather than the need for sleep, appears to decrease when people get

older. [3]

Failure to sleep results in progressively severe psychological and physical

distress. In 1965, California teenager Randy Gardner attempted to resist

sleep in an uncontrolled "experiment". As his ordeal progressed he fell into a

silent stupor, bringing into doubt whether he was actually awake in any

practical sense.

Although many believe it to be true, and while animal studies suggest it is

possible (see below), it has never been proven that total sleep deprivation will

eventually lead to death. There are no documented cases of a healthy human

dying from total sleep deprivation (excluding accidents). In carefully monitored
experiments, several normal research subjects stayed awake for 10 days.

While they all experienced cognitive deficits in memory, concentration, etc,

none of them experienced serious medical, neurological, physiological or

psychiatric problems. [4]

A 1999 University of Chicago team led by Eve Van Cauter limited a group of

lean young men to four hours of sleep per night for sixteen days. The subjects

showed decreased levels of leptin and increased levels of cortisol. The

subjects also increased their daily caloric intake by 1,000 calories. The team

discovered that the subjects' insulin and blood sugar levels resembled the

impaired glucose tolerance of prediabetics, an indication that they were no

longer properly processing carbohydrates. Studies have also linked sleep

deprivation to an increased incidence of obesity.

At Harvard Medical School, researchers have identified associations between

sleep deprivation and illnesses ranging from hypertension and heart attacks to

cancer. Poor sleepers generate increased levels of stress hormones and

show more inflammatory changes in the walls of their small blood vessels,

both of which contribute to elevated blood pressure. Because of their

exposure to light at night, night-shift workers produce less melatonin, a
hormone which not only promotes sleep but has been shown to have

cancer-prevention benefits as well.

Experiments with rats have measured the effects of long-term sleep

deprivation. In one experiment, a pair of rats were placed on a circular rotating

platform and separated by a wall. Both were instrumented with

electroencephalograms. Whenever the "subject" rat began to show signs of

sleep, the platform rotated, forcing both rats to either walk in the direction

opposite to the rotation or be forced off the platform into shallow water. The

"control" rat was allowed to sleep while the "subject" rat was awakened. After

approximately three weeks the "subject" rat became unable to regulate body

temperature; even if allowed to sleep at this point, it died shortly afterward

from septic shock. In a study by Rechtschaffen et al. (2002), rats were

sleep-deprived (eventually) for 4-6 weeks, at the beginning of which they

increased their food consumption but could not maintain their body weight.

Later, other homeostatic functions were also lost, such as thermoregulation,

and immune competence, the combination of which caused septic shock.

Other evidence that links immune function to sleep includes the enhancement

of non-REM sleep by the cytokines IL-1 and IL-2, which are actually both

released during non-REM sleep (Krueger et al., 1999).
In sleep-deprived states less extreme than that suffered by Randy Gardner,

humans display irritability, impaired cognitive function, and poor judgement.

Experiments on sleep-deprived medical trainees, for example, have shown

them less able to interpret EKGs and x-rays than their well rested peers. As

late as the early twenty-first century people thought that too little sleep could

be negated by "paying back the sleep debt". However, recent studies have

shown this to be false[citation needed].


Sleep disorders
         Main article: Sleep disorder

Disorders of sleep are broadly classified into three groups. Dyssomnias are

characterized by difficulty getting to sleep, as in primary insomnia, narcolepsy,

and restless legs syndrome. Obstructive sleep apnea, a condition that is being

diagnosed with increased frequency, may be classified either as a dyssomnia

or as an example of a parasomnia. The latter conditions involve bothersome

awakenings during sleep, and also include bruxism and sleepwalking. The

third group includes sleep disorders resulting from a number of psychiatric

problems, such as bipolar disorder, depression, or schizophrenia.

Many sleep disorders result from errors in synchronization of sleep with the

body clock. Other sleep problems are organic and cannot be resolved with
chronotherapy. One often effective solution to some kinds of insomnia

involves free-running sleep. Free-running sleep entails ignoring alarm clocks

and schedules in order to sleep when, and only when, tired. Free-running

sleep can resolve the majority of synchronization-dependent sleep disorders,

but is difficult to sustain due to the resulting loss of synchronization of sleep

with the outside world (including the day-night cycle).


Animal sleep

Sleepy lions

Animals vary widely in their amounts of sleep, from two hours a day for

giraffes to 20 hours for bats. Generally, required sleeping time decreases as

body size increases. Cats are one of the few animals that do not have most of

their sleep consolidated into one session, preferring instead to spread their

sleep fairly evenly throughout the day.
Water mammals "sleep" with alternate hemispheres of their brains asleep and

the other awake. They need to do this so they can breathe above water while

sleeping. Migratory birds also seem to sleep this way.

Even fish and fruit flies appear to have a "sleeplike" state. This alternation of

the sleeplike state and its absence is referred to as a "Basic Rest and Activity

Cycle", or BRAC. Since the modern definition of sleep is defined using EEG

criteria, and such tiny brains preclude the recording of EEG's, this may not

technically be described as sleep. However, if fruit flies are repeatedly

disturbed so that they can not rest, they have what is referred to as a "rest

rebound". This behavior is strikingly similar to that exhibited by mammals and

birds in similar conditions. As research equipment improves, the definition of

sleep may soon be revised.

Many animals hibernate in a deep sleeplike state during winter to conserve

body heat and energy. Estivation is a similar state in which other animals

hibernate to escape the heat of summer.

Cattle, horses, and sheep are unique in that they can sleep while standing,

though for cattle and sheep, REM sleep will not occur in such a position. For

REM sleep to take place, the animals must lie down. Sleeping while standing

is thus only partial sleep. However, 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 and cats sometimes make a quiet barking sound or make running

motions with their legs.


Drugs and sleep

Sleep aiding

The pharmacological approach to facilitating sleep involves the use of

depressant drugs, with the goal of inducing sleep without producing excessive

drowsiness the following morning. [5] Until the 1970s, barbiturates were

commonly prescribed for sleep, but today benzodiazepines and other newer

families of drugs with less potential for abuse and overdose are more

commonly used. Non-prescription antihistamines also have a sedative effect,

and are commonly marketed as sleep aids. Supplements of the hormone

melatonin are often used to reduce the symptoms of jet lag and delayed sleep

phase syndrome.

Sleeping pills are best prescribed only on a short-term basis and only if

symptoms of insomnia are marked enough to interfere with a patient's life.

Habitual consumption of any depressant drug to aid sleep may result in
psychological and/or physical drug dependence. Drug tolerance may also

develop, in which increasingly higher doses are required to produce the

desired effect. Physical addiction to benzodiazepines or other depressants

may lead to a withdrawal syndrome involving insomnia, anxiety, dysphoria,

and in some cases more severe problems, including seizures, delirium

tremens, and even death. Chronic use can also cause "paradoxical insomnia,"

where the drug produces the opposite of the intended effect.


Stimulant drugs

Conversely, drugs may also be used to dissuade or delay sleep. The

stimulant modafinil reduces drowsiness and is prescribed to treat narcolepsy.

It allows the user to cope with shorter sleep periods while producing only

minimal euphoric effects. The amphetamine family of stimulants by

comparison would help to keep people awake longer but also it does not

preclude the opportunity for a given person to obtain "recovery sleep" for a

longer time. Modafinil, while maintaining a longer elimination half-life, does

seem to allow sleep earlier than amphetamines. Simply put, the drug removes

the urge to fall asleep, rather than encouraging wakefulness. Modafinil also
appears to reduce, but not eliminate, the need for recovery sleep, compared

to amphetamine and placebo PMID 10607162.

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