Sleep paralysis is a common condition characterized by transient partial or total paralysis of skeletal muscles and areflexia that occurs upon awakening from sleep or less often while falling asleep. Stimuli such as touch or sound may terminate the episode, which usually has a duration of seconds to minutes. This condition may occur in normal subjects or be associated with narcolepsy, cataplexy, and hypnagogic hallucinations. The pathophysiology of this condition is closely related to the normal hypotonia that occur during REM sleep. When considered to be a disease, isolated sleep paralysis is classified as MeSH D020188.
Physiologically, it is closely related to the paralysis that occurs as a natural part of REM (rapid eye movement) sleep, which is known as REM atonia. Sleep paralysis occurs when the brain awakes from a REM state, but the bodily paralysis persists. This leaves the person fully conscious, but unable to move. In addition, the state may be accompanied by terrifying hallucinations (hypnopompic or hypnagogic) and an acute sense of danger . Sleep paralysis is particularly frightening to the individual due to the vividness of such hallucinations. The hallucinatory element to sleep paralysis makes it even more likely that someone will interpret the experience as a dream, since completely fanciful, or dream-like, objects may appear in the room alongside one's normal vision. Some scientists have proposed this condition as an explanation for alien abductions and ghostly encounters. A study by Susan Blackmore and Marcus Cox of the University of the West of England suggested that alien abductions are related to sleep paralysis rather than to temporal lobe lability.
The paralysis can last from several seconds to several minutes "after which the individual may experience panic symptoms and the realization that the distorted perceptions were false" . When there is an absence of narcolepsy, sleep paralysis is referred to as isolated sleep paralysis (ISP). "ISP appears to be far more common and recurrent among African Americans than among White Americans or Nigerian Blacks" , and is often referred to within African American communities as "the witch riding your back"