It was an ordinary night, but Salma, a 20-year-old student at The American University in Cairo, had a particularly frightening experience. She woke up, unable to move a muscle, and felt as though there were an intruder in her bedroom. She saw what appeared to be a fanged, bloody creature that looked like "something out of a horror movie," standing beside her bed.
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.