You wake — or seem to — and cannot move a muscle. There is pressure on your chest, and the unmistakable feeling that something is in the room. Sleep paralysis is one of the most frightening experiences sleep can produce, and one of the most misunderstood. It is also harmless, surprisingly common, and the origin of some of humanity's oldest monsters.

What is actually happening

During REM sleep the body is paralysed so we don't act out our dreams. In sleep paralysis, the mind wakes while that switch is still on: you are conscious, the body still obeys the dream. Because the brain is half in REM, it keeps generating dream imagery into the real bedroom — shadows, footsteps, a figure by the bed. The chest pressure is real too: REM breathing is shallow, and the effort to breathe deeply against it feels like weight. Episodes last seconds to a couple of minutes and end on their own.

The night hag: folklore got there first

Every culture described this experience long before science named it. In European folklore a witch or "night hag" sat on the sleeper's chest — the very word nightmare comes from the mare, the crushing night spirit. Elsewhere it was a demon, a ghost of the restless dead, a djinn. The dream dictionaries reflect the same imagery: dreaming of suffocation warns of a burden you've taken on; a shadowy intruder points to a fear you haven't named. The monsters differ; the experience beneath them is identical.

How to stop an episode — and prevent the next

In the moment, don't fight the whole body — that feeds the panic. Focus on the smallest movement: a fingertip, a toe, blinking hard. One small real movement breaks the paralysis. Steady your breathing and remind yourself it ends in seconds. Prevention is about protecting REM's rhythm: regular sleep hours, enough sleep, less alcohol late, managing stress — and avoiding sleeping flat on your back, the position in which most episodes occur. Frequent episodes with daytime sleepiness are worth mentioning to a doctor. And if the figure by the bed haunts you afterwards, look it up: naming the image is the oldest cure there is.