What is sleep paralysis?
Sleep paralysis (hereafter "SP") is an ostensibly harmless, but terrifying and mysterious phenomenon experienced by a small but significant portion of the population.
In an episode of SP, the person is about to drop off to sleep or has just woken up from sleep. Suddenly, the person notices that they are paralyzed - unable to move or to speak. As if that wasn't scary enough, the experience is usually accompanied by hallucinations of a threatening or "evil" presence in the room. The presence seems intent upon harming the person.
The episode ends after a few seconds or minutes, as abruptly as it began, but because of the terror it causes, it can seem to last much longer. When the episode is over, there is no lingering ill effect other than the fright: the person's mobility returns to normal and the hallucinations vanish.
How many people, and what kind of people, are affected by it?
Estimates vary; from as low as 6% to as high as 30% of survey respondents have reported at least one episode of SP in their lives. People of all races and ages and both genders have reported it.
Some researchers have reported a correlation between depressive disorders and SP experiences, including reports that antidepressants reduce the occurrence of SP episodes, but others have not found any such connection. (I myself suffer from SP but not depression; several friends suffer from depression but not SP.)
No evidence has been found to link SP to known neurological disorders such as brain tumors, aneurysms, etc. People with completely healthy nervous systems are just as likely to experience SP.
SP is usually a recurring phenomenon. Rather than everyone experiencing it once or twice, the affected portion of the population experiences it from time to time.
What causes it?
SP seems to occur because the natural paralysis which occurs during REM sleep ("rapid eye movement" sleep, during which most dreams occur) kicks in prematurely (before sleep) or doesn't switch off correctly (after waking). But no one really knows why this happens.
A number of theories have been advanced to explain SP, but unfortunately there is no certain predictor for whether a particular person will experience it, or what will trigger an episode of it. Stress, sleeping position, and bipolarity have all been linked to the condition, but none conclusively.
SP in folklore past and present
SP and its associated hallucinations have been suggested as a leading cause for reports of alien abduction, or of nocturnal attacks by "demons" in olden days. The "evil presence" hallucination is sometimes of a very "generic" sort, without specific details, but some sufferers seem to "fill in the details" with culturally-supplied data, resulting in the appearance of "aliens" or "incubi" or whatever.
Motivation
I have created this page because I, myself, occasionally suffer from SP, and I had some initial difficulties in finding information about the condition. I hope this page will provide a useful resource for people seeking information about SP. While there is very little that can be done to prevent SP at this point in our understanding, it can be comforting just to know that one is not alone in having the condition, and that it does not seem to indicate any deeper psychological or neurological disorder.
For further reading
Sleep Paralysis and Associated Hypnagogic and Hypnopompic Experiences by J. A. Cheyne, University of Waterloo.
Sleep Paralysis at the Stanford Sleep Well.
"Abduction by Aliens or Sleep Paralysis?" from Skeptical Inquirer, May/June 1998.
"Waking Up to Terror" from ABCNews.com.