What do Moby Dick, the Salem witch trials and alien abductions all have in common? They all circle back to sleep paralysis.
Less than 8 percent of the general population experiences sleep paralysis, but it is more frequent in two groups -- students and psychiatric patients -- according to a new study by psychologists at Penn State and the University of Pennsylvania.
Sleep paralysis is defined as "a discrete period of time during which voluntary muscle movement is inhibited, yet ocular and respiratory movements are intact," the researchers state in the current issue of Sleep Medicine Reviews. Hallucinations may also be present in these transitions to or from sleep.
Alien abductions and incubi and succubi, as well as other demons that attack while people are asleep, are implicated as different cultural interpretations of sleep paralysis. The Salem witch trials are now thought possibly to involve the townspeople experiencing sleep paralysis. And in the 19th-century novel Moby Dick, the main character Ishmael experiences an episode of sleep paralysis in the form of a malevolent presence in the room.
Brian A. Sharpless, clinical assistant professor of psychology and assistant director of the psychological clinic at Penn State, noted that some people who experience these episodes may regularly try to avoid going to sleep because of the unpleasant sensations they experience. But other people enjoy the sensations they feel during sleep paralysis.
"I realized that there were no real sleep paralysis prevalence rates available that were based on large and diverse samples," Sharpless said. "So I combined data from my previous study with 34 other studies in order to determine how common it was in different groups."
He looked at a total of 35 published studies from the past 50 years to find lifetime sleep paralysis rates. These studies surveyed a total of 36,533 people. Overall he found that about one-fifth of these people experienced an episode at least once. Frequency of sleep paralysis ranged from once in a lifetime to every night.