A woman lies sprawled across a bed, deep in sleep. On her chest crouches a goblin-like incubus, staring out at us. Through the curtains behind, a horse with blank, glowing eyes pushes its head into the room — a visual pun on the word "night-mare".

When Fuseli showed it at the Royal Academy in London in 1782, it caused a sensation. It wasn't a story from the Bible or classical myth, the usual subjects of serious painting. It was a picture of a feeling: the crushing, paralysed terror of a bad dream.

The image was copied and parodied endlessly, and Fuseli painted other versions. Today, people who experience sleep paralysis — waking unable to move, sometimes sensing a presence in the room — often say that this painting looks exactly like what it feels like.