Sleep scientists have made headway toward understanding sleep paralysis, reports Discover Magazine.

Sleep paralysis is distinct from nightmares, but the two do share an etymology. The Old English word maredenoted an oppressive spirit (tormenting women in male form as the incubus, and men in female form as the succubus) that sat upon the chests of sleepers, hence the suffocation. Researchers now believe the mare was inspired by sleep paralysis. The connection is clear in Henry Fuseli’s 18th-century painting, The Nightmare, where a goblinesque figure crouches atop a woman’s sprawled, inert body.

Despite the physical and psychological toll of sleep paralysis, it remains a little-understood phenomenon. Its causes are unclear, but research has linked it to stress, sleep deprivation, excessive alcohol consumption and even leg cramps. There’s also a strong correlation between sleep paralysis and other sleep disorders, like narcolepsy and obstructive sleep apnea, a condition characterized by interrupted breathing.

By itself, sleep paralysis seems to be harmless. But when people eschew their beds to escape nighttime horrors, there can be a cascade of negative health consequences linked to sleep deprivation.

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