Frontiers in Neurology reports that around 20% of people worldwide experience sleep paralysis. As the lead study authors explain, finding oneself mentally awake as the body’s voluntary muscles remain asleep can be a terrifying experience; sleep paralysis can bring with it a range of hypnagogic hallucinations.“I know firsthand how terrifying sleep paralysis can be, having experienced it many times myself,” says first study author Baland Jalal, from the University of Cambridge in the United Kingdom.