Sleep paralysis

Sleep paralysis (also hypnagogia) is a sleep disorder in which the sufferer, about to fall asleep or upon just waking, is unable to move, speak or otherwise cry out. It is always reported as being a terrifying state, accompanied by feelings of helplessness. The paralyzed state may continue for several seconds or even minutes. Many sufferers report feeling a malevolent "presence" or the sensation of being held down or otherwise forcibly restrained. Intense sensations of dread and terror are other common reported symptoms. Sometimes sufferers report loud noises accompany the attacks.

In medieval times, attacks of sufferrers of sleep paralysis may have given rise to the belief in incubi, succubi and other demons.