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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. 
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
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
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
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
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.  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.
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.
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
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
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. 
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.
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).
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
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.  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
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.
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.