Night hag
The night hag or old hag is a fantasy creature from the folklore of various peoples which is used to explain the phenomenon of sleep paralysis. It is a phenomenon during which a person feels a presence of a supernatural malevolent being which immobilizes the person as if sitting on his/her chest or the foot of his/her bed. The word "night-mare" or "nightmare" was used to describe this phenomenon before the word acquired its modern, more general meaning. Various cultures have various names for this phenomenon and/or supernatural character.
The original definition of sleep paralysis was codified by Samuel Johnson in his A Dictionary of the English Language as nightmare, a term that evolved into our modern definition. Such sleep paralysis was widely considered the work of demons, and more specifically incubi, which were thought to sit on the chests of sleepers. In Old English the name for these beings was mare or mære (from a proto-Germanic *marōn, cf. Old Norse mara), hence comes the mare part in nightmare. The word might be etymologically cognate to Greek Marōn (in the Odyssey) and Sanskrit Māra.
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