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 highly distinct modes known as non-REM and REM sleep. Although REM stands for "rapid eye movement", sleep affects other brain-body functions, including virtual paralysis of the body.
During sleep, most systems are in an anabolic state, helping to restore the immune, nervous, skeletal, and muscular systems. The internal circadian clock promotes sleep daily at night. However, sleep patterns vary among individuals. In the last century, artificial light has substantially altered sleep timing in industrialized countries.[2]
The diverse purposes and mechanisms of sleep are the subject of substantial ongoing research.[3] Sleep seems to assist with improvements in the body and mind. Research in the 21st century is investigating whether sleep is a period of maintenance for removing metabolic waste compounds from the brain.[4]
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. Sleep is sometimes confused with unconsciousness, but is quite different in terms of the thought process.
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
circadian rhythm sleep disorders
