Claim: Sleep can be disrupted by sleep paralysis.

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 . 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 '' , 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 performance and plays a large role in the function of the endocrine and immune systems . The internal circadian clock promotes sleep daily at night . The diverse purposes and mechanisms of sleep are the subject of substantial ongoing research . The advent of artificial light has substantially altered sleep timing in industrialized countries .   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 .