Sleep paralysis consists of a period of inability to perform voluntary movements either at sleep onset (called hypnogogic or predormital form) or upon awakening (called hypnopompic or postdormtal form). Sleep paralysis may also be referred to as isolated sleep paralysis, familial sleep paralysis, hynogogic or hypnopompic paralysis, predormital or postdormital
Normal sleep paralysis is thought to be due to mechanisms in the brain stem, particularly the reticular, vestibular, and oculomotor neurons, which prevent bodily movements, block sensory input and provide the forebrain with the internally generated activity that characterises brain activity during REM sleep. This is thought to be necessary to prevent the body from movements caused by dreams. Eyes however are not paralyzed by this system, and this exception was used to prove that lucid dreaming was an objectively verifiable phenomenon.
Sleep paralysis is a frightening form of paralysis that occurs when a person suddenly finds himself or herself unable to move for a few minutes, most often upon falling asleep or waking up. Sleep paralysis is due to an ill-timed disconnection between the brain and the body. The symptoms of sleep paralysis include sensations of noises, smells, levitation, paralysis, terror, and images of frightening intruders. Once considered very rare, about half of all people are now believed to experience sleep paralysis sometime during their life.
Sleep paralysis occurs when the brain is awakened from a REM state into essentially a normal fully awake state, but with the bodily paralysis still occurring. This causes the person to be fully aware, but unable to move. In addition, this state is usually accompanied by certain specific kinds of hallucinations. This state usually lasts no more than two minutes before a person is able to either return to full REM sleep or to become fully awake. Many report hallucinations during these episodes. The features of these hallucinations generally vary by individual, but some are more common to the experience than others
Little is known about the physiology of awareness during 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 depolarisation current in the nerves, which prevents the stimulation of the muscles. There is also a significant positive correlation between those experiencing this disorder frequently and those suffering from narcolepsy. However, various studies suggest that many or most people will experience sleep paralysis at least once or twice in their lives.