Ever wake up and think you see a ghost?

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.

"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.

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.

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