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Sleep paralysis is a condition that occurs when there is an ill-timed disconnection between the brain and the body when coming into or going out of sleep. This results in the sufferer finding themselves unable to move for a few seconds or several minutes, and occasionally longer.
Sleep paralysis strikes most often as a person is moving into or out of REM sleep. This is the deepest part of the sleep cycle, during which the body is disconnected from the brain, in order to prevent the dreamer physically acting out their dreams. This does, however, leave the body paralysed.
Sleep paralysis is now being studied as an explanation for terrors in the night, as many people who have experienced the phenomenon report feeling a "presence" that is often described as malevolent, threatening, or evil. An intense sense of dread and terror is very common. The presence is likely to be vaguely felt or sensed just out of sight but thought to be watching or monitoring, often with intense interest, sometimes standing by, or sitting on, the bed.
 Supernatural interpretations
Although the physical effects of sleep paralysis rarely last more than a few moments, the memories of the frightening sensations felt during an attack can last long after the episode. It is believed that sleep paralysis accounts for some (or all) of the old claims of attacks by witches, manifestations by ghosts and the more recent "reports" of nocturnal abduction by space aliens.
Sleep paralysis goes by many names, including the "old hag" in Newfoundland, based on the myth of an old witch who would sit on the chest of the victim. In the West Indies, it's "kokma", named for a ghostly baby who jumps on the sleeper's chest and attacks the throat. In China and Japan, it's "ghost pressure ("gui ya" or "kanashibari"), because a ghost sits on the sleeper's chest and assaults him. In Medieval Europe demons called incubus and succubus were said to attack women and men, respectively, usually sexually.