Sleep paralysis (SP) is a condition in which someone, just waking up or falling asleep, realizes they are unable to move. It is often accompanied by the sense of a malevolent presence in the room. Here is an excerpt from an article I wrote for After Dark(1) back in 2000:
The state of SP is not an entirely uncommon occurrence and by some estimates 15% or more of the population has an infrequent experience of it. Current research shows that SP may be more likely if an individual is stressed, jet lagged or simply taking a nap at a time other than their regular sleep period. Possibly even geomagnetic influences may play a part. Though the experience is frightening, it is not particularly dangerous or indicative of any pathology.
Reports of SP date back thousands of years. In fact, Galen, a renowned physician of the 2nd century AD thought the condition was caused by indigestion. Throughout time and across cultures a great deal of supernatural lore has accumulated in such tales as the incubus and "Old Hag," which likely have their roots in the SP phenomenon. In fact the original Anglo Saxon definition of "nightmare" comes from nicht = night and mara = incubus/succubus.
An incubus, derived from Latin, refers to a demon that visits one in their sleep, often sitting or exerting a heavy pressure on the sleeper. In medieval times, incubi were thought to have sexual intercourse with women, which could result in the birth of strange offspring. It was said that the legendary wizard Merlin was the product of such a union. David J. Hufford's fascinating book The Terror That Comes in the Night(2) delves into a wide array of folkloric associations with the condition. He shows how people of previous eras may have viewed SP events as being related to such things as witchcraft and vampirism.
It was only in the 1950's that REM (rapid eye movement-the brain state correlated with dreaming) was scientifically shown via EEG studies. One of the characteristics of REM sleep is general atonia (inability to move muscles with the exception of the eyes). It's believed that this is a protective mechanism that prevents us from acting out movements while in a dream. SP could be considered an anomalous REM state, where the brain has been simultaneously aroused into a waking mode yet still not having shut off the atonia and hypnagogic imagery associated with REM. Thus a peculiar condition occurs where the brain is mixing both dream and external reality into one perception.
Here are some additional online sources of information about sleep paralysis: stanford.edu/~dement/paralysis.html