Sleep paralysis, or more properly, sleep paralysis with hypnagogic and hypnopompic hallucinations have been singled out as a particularly likely source of beliefs concerning not only alien abductions, but all manner of beliefs in alternative realities and otherworldly creatures. Sleep paralysis is a condition in which someone, most often lying in a supine position, about to drop off to sleep, or just upon waking from sleep realizes that s/he is unable to move, or speak, or cry out. This may last a few seconds or several moments, occasionally longer. People frequently 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. On some occasions the presence may attack, strangling and exerting crushing pressure on the chest. People also report auditory, visual, proprioceptive, and tactile hallucinations, as well as floating sensations and out-of-body experiences (Hufford, 1982). These various sensory experiences have been referred to collectively as hypnagogic and hypnopompic experiences (HHEs). People frequently try, unsuccessfully, to cry out. After seconds or minutes one feels suddenly released from the paralysis, but may be left with a lingering anxiety. Extreme effort to move may even produce phantom movements in which there is proprioceptive feedback of movement that conflicts with visual disconfirmation of any movement of the limb. People may also report severe pain in the limbs when trying to move them. Several recent surveys including our own suggest that between 25-30% of the population reports that they have experienced at least a mild form of sleep paralysis at least once and about 20-30% of these have had the experience on several occasions. A few people may have very elaborate experiences almost nightly (or many times in a night) for years. Aside from many of the very disturbing features of the experience itself (described in succeeding sections) the phenomenon is quite benign. It was thought in the past that it was a significant part of the so-called "narcoleptic tetrad", but recent surveys of non-clinical populations, such as ours, suggest that the prevalence may be as high among the general population as among diagnosed narcoleptics.
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