Were you sleepy? Did you doze off into a state of partial awareness where you may have dreamt this figure up?rnrn"She later explained her experience to researchers who were conducting a survey about sleep paralysis, a common but somewhat unexplained phenomenon in which a person awakens from sleep but feels unable to move. Up to 40 percent of people report experiencing sleep paralysis at some point in their lives, and a few, like Salma, hallucinate shadowy intruders hovering over them.rnrn"Sleep paralysis can be a very frightening experience for some people, and a clear understanding of what actually causes it would have great implications for people who suffer from it," said Baland Jalal, a neuroscientist at the University of California, San Diego."rnrn"Researchers say that sleep paralysis happens when a person awakens during a stage of sleep known as rapid eye movement (REM). People in this stage of sleep are usually dreaming, but their muscles are nearly paralyzed, which might be an evolutionary adaptation that keeps people from acting out their dreams.rnrnIt is harder to explain why a subset of people who experience sleep paralysis feel a menacing figure in their room or pressing on their chests. [Senses and Non-Sense: 7 Odd Hallucinations]"rnrnSo, were you sleepy? Were you just waking up, were you falling asleep?