Maybe you've heard about it from a friend, or maybe you've experienced it yourself. But either way you slice it, the concept of sleep paralysis -- not being able to move your limbs either before drifting off to sleep, or right after waking, even though you're still able to see and breathe -- is eerie.
And now, a new study in Sleep Medicine Reviews is cluing us in to who's most likely to experience this sleep phenomenon: students and psychiatric patients.
Brian A. Sharpless, a clinical assistant professor of psychology at Penn State, analyzed the results of 34 previous studies that were published over the last 50 years, involving 36,533 people. He found in his research that sleep paralysis is experienced by 7.6 percent of the general population. However, it occurs in 28 percent of students and 32 percent of psychiatric patients (they experienced at least one incident of sleep paralysis).
Specifically, sleep paralysis is more common among people who have panic disorder, with 35 percent of them reporting experiencing at least one episode, according to the study.
Sharpless also found that sleep paralysis seems to be more prevalent among non-white ethnic groups.
"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," Sharpless said in a statement.
In the past, sleep paralysis was pegged to being abducted by aliens or being attacked by demons or evil presences. But these days, it's understood that it actually has to do with our body's transition between REM (rapid eye movement) sleep and NREM (non-rapid eye movement) sleep, WebMD reported.
NREM sleep occurs first and takes up to 75 percent of your overall sleep time. During NREM sleep, your body relaxes and restores itself. At the end of NREM, your sleep shifts to REM. Your eyes move quickly and dreams occur, but the rest of your body remains very relaxed. Your muscles are "turned off" during REM sleep. If you become aware before the REM cycle has finished, you may notice that you cannot move or speak.
Typically during sleep paralysis, people experience some sort of hallucination, whether it's a sense of an intruder, a sense of physical or sexual assault, levitation or an out-of-body experience.
A sense of pressure on the chest, or even a sensation of choking, can also occur, WebMD pointed out.
Sleep paralysis is most common when you're lying on your back, not following a normal sleep pattern (whether from jet lag, school or using a substance like alcohol or caffeine) or if you're extra stressed, Scientific American reported. The phenomenon is considered a symptom of narcolepsy, though it can also occur in people without the sleep disorder.