Sleep paralysis is now being studied as an explanation for terrors in the night, which have been experienced by people across all cultures and for thousands of years. If one is looking for a purely physical and scientific explanation for these terrible nightmares, this one works quite well. For some it will offer relief but for others, doubt.
Sleep paralysis is a condition in which someone, most often lying in a supine (face up) position, about to drop off to sleep, or just upon awaking 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.
There are a number of historical and urban cultural myths, which can be, somewhat, explained by this experience. The Incubus, which appears in ancient literature, is one such example. In the book Incubus by Kiessling, It was described as half man half beast, attacking in the night. The word night "mare" has been derived from the word incubus. In Greek it was ephialtes, in Latin incubus, in German mar/mare, in Old English maire, Old Norse mara, Old Irish mar/mor, and all mean "one who leaps on, oppresses or crushes."
The demon of the night has also been called 'The Old Hag' a description and myth coming out of several cultures. The Old Hag was described in Shakespeare's Romeo and Juliet.
The Old Hag attack is most closely associated with extreme pressure on the chest while sleeping on your back. People may also feel like they are being choked or even bitten. In his research, Al Cheyne of the University of Waterloo has discovered that between 25 and 30 per cent of the population reports that they have experienced at least a mild form of sleep paralysis at least once. It most often has an adolescent onset but can begin at any age.
Cheyne believes sleep paralysis to be an hallucination created by physical things occurring in the body as a result of a dysfunction or malfunction of the normal R.E.M. state of sleep. This malfunction may be brought on by life stressors or sleep deprivation. Cheyne has also discovered a much higher incidence of sleep paralysis for those who sleep on their back. He has found that changing sleep position can reduce the incidence of these nightmares. He also suggests getting up and physically moving around after having an episode as several can occur in one night.