Sleep paralysis occurs when you awaken to find that you cannot move or speak. This experience can be so terrifying that many deem it a visit from demons, known from East Asian cultures to European ones as “night hags.”

Not that fantastical creatures aren’t a good explanation for this phenomenon, but for those who prefer a more reality-based rundown, here’s what actually happens during a bout of sleep paralysis: your brain stem, which is responsible for regulating your sleep pattern, shuts your muscles down once you enter the REM phase, so that you don’t act on your dreams (clever brain!). Sometimes, however, it’s a little buggy—just like any other complex system—and forgets to unfreeze your body even though you’ve regained consciousness. This results in a few seconds, or sometimes minutes, of your mind feeling trapped in your own body.

Dive into the surprising world of the human brain with the blinks to The Tale of the Dueling Neurosurgeons by Sam Kean.