incubare

The Latin verb incubare meant 'to lie upon' or 'to brood upon,' from in- ('upon') and cubare ('to lie down'). The incubus (plural incubi) was the demon that lay upon sleeping people, causing the sensation of weight on the chest, inability to move, and terrifying visions. The female counterpart was the succubus, from succubare ('to lie beneath'). Both names describe the demon's position relative to the victim.

Sleep paralysis is the medical reality behind the incubus. During REM sleep, the body is temporarily paralyzed to prevent sleepers from acting out dreams. Sometimes the paralysis persists into wakefulness — you wake up unable to move, often with a sensation of pressure on the chest and the perception of a malevolent presence in the room. An estimated 8% of the population experiences this at least once.

Every culture has an incubus figure. The Japanese kanashibari. The Newfoundland 'Old Hag.' The Chinese gui ya chuang ('ghost pressing bed'). The Turkish karabasan ('dark presser'). The experience is universal. The explanation varies. Medieval Christian theology chose the demon because it fit the framework: sin invited the incubus, and prayer expelled it. The victim was always partially blamed.

English borrowed incubus from Latin by the 1200s. The word 'nightmare' itself preserves the same concept — the Old English mare was a female spirit that sat on sleepers' chests. 'Night-mare' was not a bad dream. It was the night visitor, the thing that rode you. The incubus and the nightmare were the same creature in different languages.