Sleep paralysis is a condition characterized by temporary paralysis of the body shortly after waking up (known as hypnopompic paralysis) or, less often, shortly before falling asleep (known as hypnagogic 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 aware, but unable to move. In addition, the state may be accompanied by hypnagogic hallucinations. More often than not, sleep paralysis is believed by the person affected by it to be no more than a dream. This explains many dream recountings which describe the person lying frozen and unable to move. 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.
Possible causesSleep 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 dreamt activity (e.g. preventing a sleeper from flailing his legs when dreaming about running). Many people who commonly enter sleep paralysis also suffer from narcolepsy. Especially in African-Americans panic disorder often co-occurs with sleep paralysis. However, various studies suggest that many or most people will experience sleep paralysis at least once or twice in their lives. Some reports read that various factors increase the likelihood of both paralysis and hallucinations. These include: 
Sleeping in an upwards supine position
Irregular sleeping schedules; naps, sleeping in, sleep deprivation
Sudden environmental/lifestyle changes
A lucid dream that immediately precedes the episode. Also conscious induction of sleep paralysis is a common technique to enter a state of lucid dreams, also known as WILD .
Artificial sleeping aids, ADD medications and/or antihistamines
Recent use of hallucinogenic drugs
TreatmentDuring paralysis episodes, patients may be advised to try moving the facial muscles and moving eyes from one side to the other. This may hasten the termination of the attack. Clonazepam is highly effective in the treatment of sleep paralysis. The initial dose is 0.5 mg at bedtime, while an increase to 1 mg per night might be necessary to maintain potency. Anecdotal reports indicate SSRIs such as fluoxetine markedly decrease the incidence of sleep paralysis. Several people who have been both on and off SSRIs have reported corresponding decreases and increases in sleep paralysis episodes. Others report no effects at all.
Cultural referencesComplete references to many cultures are given in the References section
In Vietnamese, sleep paralysis is referred to as "ma de", meaning "held down by a ghost". Most people in this culture believe that a ghost has entered your body, causing the paralysis state.
In Japanese, sleep paralysis is referred to as kanashibari (, literally "bound or fastened in metal," from kane "metal" and shibaru "to bind, to tie, to fasten"). This term is occasionally used by English speaking authors to refer to the phenomenon both in academic papers and in pop psych literature.
In Hungarian folk culture sleep paralysis is called "lidércnyomás" ("lidérc pressing") and can be attributed to a number of supernatural entities like "lidérc", "boszorkány" (witch), "tündér" (fairy) or "ördögszerető". The word "boszorkány" itself stems from the Turkish root "bas-", meaning "to press".
Kurdish people call this phenomenon a "mottaka", they believe that some one, in a form of a ghost or perhaps an evil spirit, turns up on top the of the person in the middle of the night and suffocates him/her. Apparently this happens usually when some one has done something bad.
In New Guinea, people refer to this phenomenon as "Suk Ninmyo", believed to originate from sacred trees that use human essence to sustain its life. The trees are said to feed on human essence during night as to not disturb the human's daily life, but sometimes people wake unnaturally during the feeding, resulting in the paralysis.
In Turkey this is called "karabasan" ("black-pressing"). It is believed that it is a creature which attacks people in their sleep.
In Mexico, it's believed that sleep paralysis is in fact the spirit of a dead person getting on the person and impeding movement, calling this "se me subió el muerto" (the dead person got on me).
Ogun Oru is a traditional explanation for nocturnal disturbances among the Yoruba of Southwest Nigeria; ogun oru (nocturnal warefare) involves an acute night-time disturbance that is culturally attributed to demonic infiltration of the body and psyche during dreaming. Ogun oru is characterized by its occurrence, a female preponderance, the perception of an underlying feud between the sufferer's earthly spouse and a ;spiritual' spouse, and the event of bewitchment through eating while dreaming. The condition is believed to be treatable through Christian prayers or elaborate traditional rituals designed to exorcise the imbibed demonic elements. 
Several studies have shown that African-Americans may be predisposed to isolated sleep paralysis also known as "the witch is riding you," or "the haint is riding you."  In addition, other studies have shown that African-Americans who have frequent episodes of isolated sleep paralysis, i.e., reporting having one or more sleep paralysis episodes per month coined as "sleep paralysis disorder," were predisposed to having panic attacks.  This finding has been replicated by other independent researchers  
Sleep paralysis in literature, art and music
(1605) Miguel de Cervantes makes mention of the phenomenon in Don Quixote when a tavern wench jumps into the bed of the soundly sleeping Sancho Panza, who, started, and feeling a prodigious weight upon him, thought he was labouring under the nightmare.
(1851) There is a particularly fascinating account of sleep paralysis in Herman Melville's novel Moby-Dick. Chapter 4 (The Counterpane) is an account of Ishmael's meditation on an episode of sleep paralysis in the middle of which he could not distinguish the difference between Queequeg's arm and the quilt. Indeed, he could not even distinguish the difference between his own body and his surroundings. He then recalls an earlier episode of sleep paralysis from his childhood, which he determines was the precise moment he discovered the feeling of "otherness" of his own body with respect to his surroundings.
(1936) An account can also be found in Ernest Hemingway's The Snows of Kilimanjaro, in which death approaches and sits upon the narrator's chest so that he cannot breathe.
(1969) The main character in Kingsley Amis' novel The Green Man also suffers from the affliction.
(1975) Maxine Hong Kingston recounts an episode in her book The Woman Warrior where her mother, Brave Orchid, suffers a night of sleep paralysis in the "Haunted Room", in which she claims she battles a "Sitting Ghost".
In Zimbabwean Shona culture the word Madzikirira is used referring to the fact that something will really be pressing you down and mostly they refer to the spiritual world where some other spirit especially evil one will be trying to use you. The people believe that witches can only be people of close relations to be effective and hence a witch can try to use your spirit to bewitch your relatives.
(2001) The progressive rock band Dredg explores the different aspects of sleep paralysis, on their album El Cielo. The booklet with El Cielo contains letters written by sufferers of sleeping disorders with descriptions of various experiences with or relating to sleep paralysis. Singer Gavin Hayes incorporates and expands upon the material found in the booklet for the lyrics to the album; all of the songs on the album (except the instrumentals) contain snippets of the text in the booklet.
(2006) Experimental band Fear Before the March of Flames talk about the struggles of dealing with constant sleep paralysis on their album "The Always Open Mouth". One of the songs is even called "Drowning the Old Hag".
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