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sleep paralysis
Physiologically, it is closely related to the paralysis that occurs as a natural part of REM (rapid eye movement) sleep, which is known as REM atonia. Sleep paralysis occurs when the brain awakes from a REM state, but the bodily paralysis persists. This leaves the person fully conscious, but unable to move.In addition, the state may be accompanied by terrifying hallucinations (hypnopompic or hypnagogic) which cause an acute sense of danger [2]. Sleep paralysis is particularly frightening to the individual due to the vividness of such hallucinations[3]. The hallucinatory element to sleep paralysis makes it even more likely that someone will interpret the experience as a dream, since completely fanciful, or dream-like, objects may appear in the room alongside one's normal vision. Some scientists have proposed this condition as a theory for alien abductions and ghostly encounters.