Ive read on this alot since i started having it happen. I dont buy into the whole ghost, and spiritual nonsense so that explanation wont work for me. I dont mean this to be offensive to anyone that does though. I said it wrong that your brain wakes up from REM but your body persists in it being unable to move. Ive been known to wake up from it happening and smile like that was kinda cool then just go back to sleep.
Physiologically, sleep paralysis 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. 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".[5] As the correlation with REM sleep suggests, the paralysis is not entirely complete; use of EOG traces shows that eye movement can be instigated during such episodes.[6] When there is an absence of narcolepsy, sleep paralysis is referred to as isolated sleep paralysis (ISP).[7] "ISP appears to be far more common and recurrent among people of African descent than among whites or Nigerian Africans",[5] and is often referred to within African communities as "the Devil on your back."[8][9][10]
In addition, the paralysis state may be accompanied by terrifying hallucinations (hypnopompic or hypnagogic) and an acute sense of danger.[11] Sleep paralysis is particularly frightening to the individual because of the vividness of such hallucinations.[7] 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.[12] A study by Susan Blackmore and Marcus Cox (the Blackmore-Cox study) of the University of the West of England supports the suggestion that reports of alien abductions are related to sleep paralysis rather than to temporal lobe lability.[13]