Sleep paralysis: A frightening form of paralysis that occurs when a person suddenly finds himself or herself unable to move for a few minutes, most often upon falling asleep or waking up. Sleep paralysis is due to an ill-timed disconnection between the brain and the body.

The symptoms of sleep paralysis include sensations of noises, smells, levitation, paralysis, terror, and images of frightening intruders. Once considered very rare, about half of all people are now believed to experience sleep paralysis sometime during their life.

Sleep paralysis strikes as a person is moving into or out of REM (rapid eye movement) sleep, the deepest part of sleep. During REM sleep the body is largely disconnected from the brain leaving the body paralyzed. Sleep paralysis is the result of premature (or persistent) mind-body disconnection as one is about to enter into (or exit from) REM sleep.

Sleep paralysis occurs most often after jet lag or periods of sleeplessness that interrupt the normal REM patterns. It affects both sexes equally and occurs at all ages but is most common in teenagers. Sleep paralysis can be familial and may be genetic (inherited) in some cases.

An attack of sleep paralysis is usually harmless and self-limited. It tends to be over in a minute or two as soon as the brain and body re-establish connections and the person is able to move again. However, the memory of the terrifying sensations felt during sleep paralysis can long endure. (Some scholars believe that sleep paralysis may account for some of the old claims of attacks by witches and the more recent "reports" of nocturnal abduction by space aliens.)

A rare fatal form of sleep paralysis may, it is thought, underlie the cases of healthy teenagers, mainly in Southeast Asia, who die in their sleep, sometimes after fighting for breath but without thrashing around.

Sleep paralysis goes by a number of names, including the "old hag" in Newfoundland (for an old witch thought to sit on the chest of the paralyzed sleeper), "kokma" in the West Indies (for a ghost baby who jumps on the sleeper's chest and attacks the throat), "kanashibari" in Japan and "gui ya" or ghost pressure in China (because a ghost is believed to sit on and assault the sleeper). Medically, sleep paralysis is sometimes called waking paralysis, predormital (before-sleep) paralysis, postdormital (after-sleep) paralysis, and REM sleep atonia.



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