It is estimated that up to 40 percent of people will experience some form of sleep paralysis involving the inability to move any part of their body. During this state only the eyes can open and a number of vivid sensory experiences (hallucinations) may be experienced. Typically, these will involve a sense of some feared intrusion, or pressure on the chest accompanied by a sense of choking, or some out of body experience such as floating or flying.
Sleep paralysis is a state that occurs just before falling asleep or just upon awakening. Moreover, this can be accompanied by hypnagogic (just before sleep) or hypnopompic (just upon awakening) experiences of a particularly vivid sensory nature.
Various lines of research have concluded that the combination of sleep paralysis and hypnagogic states explains the worldwide accounts of spirit possession, ghostly visitations, alien abductions and incubus/succubus assaults. The accounts from people who experience such terrors tend to vary according to culture, but the physical effects of such perceived assaults are remarkably uniform in their descriptions. A common account is the sense of some evil presence sitting heavily on the chest and strangling the person who is incapable of defending themselves. Culturally, this apparition may be described in the form of a vampire, an old hag or witch, or a more ghostly or spiritual presence.
The explanation for the physical nature of such experiences lies partly in the known mechanisms of REM (Rapid Eye Movement) states associated with the sleep cycle. During REM, bursts of activation originating in the brain stem pass through areas of the brain associated with both emotion and the fear response. In the fully awake state potential dangers are rapidly assessed confirmed or dismissed, but within a state of sleep paralysis, apprehension that would normally last milliseconds can drag on into seconds or even minutes.
Similarly, breathing rates can be quite variable during REM. It has been suggested that thoracic breathing is lower during REM, so as with any attempt to move muscles during sleep paralysis, attempts to control breathing will be unsuccessful. The sense of resistance that follows any such attempt will be interpreted as pressure and choking.
Cheyne, J.A., Rueffer, S.D., Newby-Clark, I.R. (1999) Hypnagogic and Hynopompic Hallucinations during Sleep Paralysis: Neurological and Cultural Construction of the Night-Mare. Conciousness and Cognition 8, 319-337.