Sleep is the state of natural
rest observed in humans and
throughout the animal kingdom,
in all mammals and birds, and
in many reptiles, amphibians,
and fish.In humans, other
mammals, and many other
animals that have been studied
â?? such as fish, birds, ants,
and fruit-flies â?? regular
sleep is necessary for
survival. The c
more

Sleep is the state of natural
rest observed in humans and
throughout the animal kingdom,
in all mammals and birds, and
in many reptiles, amphibians,
and fish.In humans, other
mammals, and many other
animals that have been studied
â?? such as fish, birds, ants,
and fruit-flies â?? 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 generally
characterized by a reduction
in voluntary body movement,
temporary blindness, decreased
reaction to external stimuli,
loss of consciousness, a 70%
reduction in hearing capacity,
an increased rate of anabolism
(the synthesis of cell
structures), and a decreased
rate of catabolism (the
breakdown of cell
structures).Sleep StagesThe
measurement of eye movement
during sleep is used to divide
sleep into two broad types:
rapid eye movement (REM) and
non-rapid eye movement (NREM)
sleep. Each type has a
distinct set of associated
physiological, neurological
and psychological features.
Sleep proceeds in cycles of
REM and NREM phases. In
humans, this cycle is
approximately 90--110 minutes.
Each phase 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.In REM, the brain is
active and the body inactive,
and this is when most dreaming
occurs. REM sleep is
characterized by an
electroencephalography (EEG)
that has low voltage and 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, it is
usually REM sleep.In NREM
sleep, the body is active and
the brain inactive, and there
is relatively little dreaming.
Non-REM encompasses four
stages; stages 1 and 2 are
considered 'light sleep', and
3 and 4 'deep sleep'. They are
differentiated solely using
EEG, unlike REM sleep which is
characterized by rapid eye
movements and relative absence
of muscle tone. There are
often limb movements, and
parasomnia sleep walking
occurs in non-REM sleep.A
cyclical alternating pattern
may sometimes be observed
during a stage.Rechtschaffen
and Kales originally outlined
the criteria for staging sleep
in 1969. The American Academy
of Sleep Medicine (AASM)
updated the staging rules in
2007. Stage 4 Sleep. EEG
highlighted by red box. REM
Sleep. Eye movements
highlighted by red line. Both
REM sleep and NREM sleep
stages 3 and 4 are
homeostatically driven; that
is, if a human is selectively
deprived of one of these, it
rebounds once the person is
allowed to sleep. This
suggests that both are
essential in the sleep process
and its many
functions.NREMNREM accounts
for 75--80% of total sleep
time in normal human adults,
and consists of four stages
according to the 2007 AASM
standards:Active Sleep is a
phase of sleep in newborns
that appears similar to REM
sleep in adults. Neonatal
sleep depends on age and is
sometimes divided into Active
Sleep, Quiet Sleep, and Wake.
This is less specific than the
classification of adult's
sleep, and is often based on
behavioral criteria because it
is technically difficult to
record an EEG from a newborn.
Some scientists suggest that
Active Sleep is similar to REM
sleep. Others say it is an
entirely different state, and
point to differences between a
developing central nervous
system and a mature one. Sleep
RegulationThe cycle of sleep
and wakefulness is regulated
by the brain stem, thalamus,
external stimuli, and various
hormones produced by the
hypothalamus. Three processes,
influenced by hormonal,
neurological, and
environmental factors,
underlie sleep regulation:A
homeostatic process determined
by prior sleep and
wakefulness, determines 'sleep
need.' A circadian process
determines periods of high and
low sleep propensity, and high
and low REM sleep propensity.
An ultradian process The
interrelationships and
relative importance of each
process and system remain
uncertain.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, and decreases
during sleep. Researchers
believe that its accumulation
encourages sleep. The
stimulant properties of
caffeine are attributed to its
negating the effects of
adenosine. But 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.

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