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.
When looking at specific groups, 28 percent of students reported experiencing sleep paralysis, while nearly 32 percent of psychiatric patients reported experiencing at least one episode. People with panic disorder were even more likely to experience sleep paralysis, and almost 35 percent of those surveyed reported experiencing these episodes. Sleep paralysis also appears to be more common in non-Caucasians.
"Sleep paralysis should be assessed more regularly and uniformly in order to determine its impact on individual functioning and better articulate its relation to other psychiatric and medical conditions," said Sharpless.
He looked at a broad range of samples, and papers were included from many different countries.
People experience three basic types of hallucinations during sleep paralysis -- the presence of an intruder, pressure on the chest sometimes accompanied by physical and/or sexual assault experiences and levitation or out-of-body experiences.
Up to this point there has been little research conducted on how to alleviate sleep paralysis or whether or not people experience episodes throughout their lives.
"I want to better understand how sleep paralysis affects people, as opposed to simply knowing that they experience it," said Sharpless. "I want to see how it impacts their lives." Sharpless hopes to look at relationships between sleep paralysis and post-traumatic stress disorder in the future.