Sleep is a naturally recurring state of mind and body, characterized by altered
consciousness, relatively inhibited sensory activity, inhibition of nearly all
voluntary muscles, and reduced interactions with surroundings.
[1] It is distinguished from
wakefulness by a decreased ability to react to
stimuli, but is more easily reversed than the state of being
comatose. Sleep occurs in
repeating periods, in which the body alternates between two distinct modes:
REM sleep and
non-REM sleep. Although REM stands for "rapid eye movement", this mode of sleep has many other aspects, including virtual
paralysis of the body. A well-known feature of sleep is the
dream, an experience typically recounted in
narrative form, which resembles waking life while in progress, but which usually can later be distinguished as fantasy.
During sleep, most of the
body's systems are in an
anabolic state, helping to restore the immune, nervous, skeletal, and muscular systems; these are vital processes that maintain mood, memory, and cognitive function, and play a large role in the function of the
endocrine and
immune systems.
[2] The internal
circadian clock promotes sleep daily at night. The diverse purposes and mechanisms of sleep are the subject of substantial ongoing research.
[3] The advent of
artificial light has substantially altered sleep timing in industrialized countries.
[4]
Humans may suffer from various
sleep disorders, including
dyssomnias such as
insomnia,
hypersomnia,
narcolepsy, and
sleep apnea;
parasomnias such as
sleepwalking and
REM behavior disorder;
bruxism; and
circadian rhythm sleep disorders.