A 'nightmare' was originally a female demon that sat on sleeping people's chests
The 'mare' in nightmare comes from Old English and Old Norse 'mare' — a malevolent female spirit believed to sit on people's chests while they slept, causing terrifying dreams, chest pressure, and paralysis. This exactly describes sleep paralysis (which many people now understand as a neurological event). The 'night' was added later to distinguish it from the regular word 'mare'. In German the creature is called 'Nachtmahr'; in Danish, 'Nat-mare'. The supernatural explanation for sleep paralysis — a demon sitting on you — appears independently in cultures worldwide, including in Japan ('kanashibari') and China ('ghost oppression').
Every culture independently invented a female demon to explain the same neurological event — sleep paralysis. That these explanations converge across unconnected cultures suggests how frightening and universal the experience is, and how the supernatural explanation felt necessary before neuroscience existed.
“'Nightmare' originally meant a female demon that sat on your chest while you slept — describing what we now call sleep paralysis. Every culture independently invented a demon to explain the same neurological event. 👻 #OddlyHuman”