The Demon Sitting on Your Chest Has a Medical Name
Sleep paralysis has an unfortunate design.
Your mind wakes before your body receives the same notification.
During rapid eye movement sleep, or REM sleep, the brain suppresses much of the body's skeletal muscle activity. This is useful. Without it, dreams involving running away from wolves would be considerably harder on bedroom furniture.
Usually this paralysis disappears as you wake. During sleep paralysis, elements of REM sleep and waking consciousness overlap. You become aware of your surroundings while voluntary movement is still temporarily inhibited.
That alone would be unpleasant.
The brain occasionally improves matters by adding a visitor.
People experiencing sleep paralysis commonly report sensing a presence in the room. Some see a human figure. Others describe something sitting on their chest, approaching the bed or watching from nearby. Auditory hallucinations can occur too.
Across cultures and centuries, these experiences have been interpreted as demons, witches, spirits, shadow people, supernatural attacks and various regional creatures with a disturbing interest in bedrooms.
The experience is real. The creature is another matter.
Sleep researchers classify the threatening-presence experience as an intruder-type hallucination associated with sleep paralysis. Another pattern, sometimes called the incubus phenomenon, involves a feeling of pressure on the chest and difficulty breathing.
When someone wakes paralysed, frightened, breathing strangely and apparently sees a figure standing beside them, "I am experiencing an unusual REM transition" is understandably not the first interpretation that comes to mind.
The supernatural explanation also has one enormous psychological advantage. There really does appear to be something there.
Knowing what sleep paralysis is does not necessarily make an episode pleasant.
But it does transform the mystery.
For thousands of years, people may have been waking in the same biological state and giving the visitor different names.
The demon changed. The bedroom changed. The nervous system did not.