Sleep paralysis is a condition that may occur in normal subjects or be associated with narcolepsy, cataplexy, and hypnagogic hallucinations. The pathophysiology of this condition is closely related to the normal hypotonia that occur during REM sleep.[1] When considered to be a disease, isolated sleep paralysis is classified as MeSH D020188.[2] Some evidence suggests that it can also, in some cases, be a symptom of migraine.[3][4]
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Symptoms and characteristics
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] When there is an absence of narcolepsy, sleep paralysis is referred to as isolated sleep paralysis (ISP).[6] "ISP appears to be far more common and recurrent among blacks than among whites or Nigerian blacks",[7] and is often referred to within black communities as "the Devil on your back"[8][9]
In addition, the paralysis state may be accompanied by terrifying hallucinations (hypnopompic or hypnagogic) and an acute sense of danger.[10] Sleep paralysis is particularly frightening to the individual due to the vividness of such hallucinations.[11] 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 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]
[edit] Possible causes
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.[citation needed] However, some have suggested that it may be linked to post-synaptic inhibition of motor neurons in the pons region of the brain.[citation needed] In particular, low levels of melatonin may stop the depolarization current in the nerves, preventing the stimulation of the muscles and any consequent enactment of the dream activity by the body (e.g. preventing a sleeper from flailing his legs when dreaming about running).[citation needed]
Several studies have concluded that many or most people will experience sleep paralysis at least once or twice in their lives.[citation needed] A study conducted by Sedaghat F. et al. has investigated the prevalence of sleep paralysis among Iranian medical students. 24.1% of students reported experiencing sleep paralysis at least once in their lifetime. The same result was reported among Japanese, Nigerian, Kuwaiti, Sudanese and American students.[14]
Many people who commonly enter sleep paralysis also suffer from narcolepsy. In non-Nigerian blacks, panic disorder occurs with sleep paralysis more frequently than in Caucasians.[15] Some reports read that various factors increase the likelihood of both paralysis and hallucinations. These include:[16]
* Sleeping in a face upwards or supine position
* Irregular sleeping schedules; naps, sleeping in, sleep deprivation
* Increased stress
* Sudden environmental/lifestyle changes
* A lucid dream that immediately precedes the episode.