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 body 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) and an acute sense of danger. Sleep paralysis is particularly frightening to the individual due to the vividness of such hallucinations. 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 an explanation for alien abductions and ghostly encounters. A study by Susan Blackmore and Marcus Cox of the University of the West of England suggested that alien abductions are related to sleep paralysis rather than to temporal lobe lability.
The paralysis can last from several seconds to several minutes "after which the individual may experience panic symptoms and the realization that the distorted perceptions were false". When there is an absence of narcolepsy, sleep paralysis is referred to as isolated sleep paralysis (ISP). "ISP appears to be far more common and recurrent among African Americans than among White Americans or Nigerian Blacks", and is often referred to within African American communities as "the witch riding your back".
Symptoms of sleep paralysis can be either one of the following or a combination:
Paralysis: this occurs after waking up or shortly before falling asleep. The person cannot move any body part, and only has minimal control over blinking and breathing. This paralysis is the same paralysis that occurs when dreaming. The brain paralyzes the muscles to prevent possible injury during dreams, as some body parts may move during dreaming. If the person wakes up suddenly, the brain may still think that it is dreaming, and sustains the paralysis. Hallucinations: Images or speaking that appear during the paralysis. The person may think that someone is standing beside them or they may hear strange sounds. These may be dreamlike, possibly causing the person to think that they are still dreaming. Often it is reported as feeling a weight on one's chest, as if being underneath a person or heavy object. These symptoms can last from mere seconds to several minutes (although they can feel like much longer) and can be frightening to the person. There may be some body movement, but it is very unlikely and hard for a person to accomplish.
Sleep paralysis occurs during REM sleep, thus preventing the body from manifesting movements made in the subject's dreams. Very little is known about the physiology of sleep paralysis. However, some have suggested that it may be linked to post-synaptic inhibition of motor neurons in the pons region of the brain. In particular, low levels of melatonin may stop the depolarization current in the nerves, which prevents the stimulation of the muscles, to prevent the body from enacting the dream activity (e.g. preventing a sleeper from flailing his legs when...