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All sleep disorders are difficult. But few are as soul-shaking as sleep paralysis. During an episode, your brain is essentially awake, but you’re frozen from the neck down and unable to speak. Your eyes can see, but you have little control over how to use them or on what they focus. In this limbo between sleep and waking, some people feel crushing pressure on their chests or might hallucinate that hideous beasts or malevolent spirits are in the room with them, but they’re powerless to defend themselves. They must watch and wait; they can't even scream. 

“I would sometimes wake up and one or more ‘spirits’ would be swarming around me trying to get inside and inhabit my body,” says Elise Thompson, an editor and blogger in Los Angeles who says she experienced sleep paralysis episodes mostly between the ages of 8 and 16. “They were wispy like smoke, kind of the stereotypical human-like head and reaching arms that dissolve into a long trail, like they are made out of the wind. Other times I’d wake up and an old woman would be sitting by my bed.”

Sleep paralysis occurs when your brain moves out of atonia, a purposeful paralysis, but your body hasn’t yet caught up.

Estimates vary widely about how many people have experienced sleep paralysis. It affects 7.6 percent of the general population but 28 percent of students, according to a 2011 study published in Sleep Medicine Reviews. Another study, however, published in 2001 concluded that nearly 40 percent of people who responded to a survey said they’d experienced sleep paralysis at least once. It’s said to affect teens and young adults more frequently and is more common among women and African Americans, particularly those with panic disorder. But it’s not associated with or considered to be a mental health disorder in and of itself. 

Why does it happen? Well, during REM sleep, something called atonia calms your muscles and keeps you from flailing around and injuring yourself as you dream. Sleep paralysis occurs when your brain moves out of this phase but your body hasn’t yet caught up. The exact theory of what causes sleep paralysis is unknown, but one theory about why terror often sets in during sleep paralysis is that the amygdala, which some scientists say is the threat recognition center of the brain, is active during sleep paralysis and anxiety results when the body can’t respond. 

“The amygdala … can trigger the ‘fight or flight’ response,” says Dr. Michael J. Peterson, M.D., Ph.D. associate professor and director of Hospital Psychiatric Services at the University of Wisconsin School of Medicine and Public Health and author of a recent review of a book about sleep paralysis. “I don’t know that the amygdala is particularly active during REM, but would expect a fear response during REM [such as] a nightmare or during a sleep paralysis episode.”

Despite a centuries-long fascination with this common and terrifying sleep disorder, scientific understanding of it is still fairly slim. Here’s what researchers have concluded about sleep paralysis so far. 

1. Sleep paralysis causes three general types of hallucinations.

In a paper published in Consciousness and Cognition in 1999, researchers noted three types of common hallucinations of sleep paralysis sufferers: the intruder, the incubus and “unusual body experiences,” such as floating, levitating and out-of-body experiences. They also found that people’s descriptions of sleep paralysis-related hallucinations are “remarkably consistent across time and cultures and consistent also with known mechanisms of REM states.”

Intruder hallucinations, such as the ones Thompson experienced, are fairly common, as are feelings of being crushed or suffocated, which researchers in the aforementioned study associated with the incubus category. 

Interestingly, one study published in the Journal of Sleep Research in 2005 concluded that participants considered “novices” in the sleep paralysis-experience realm were more likely to have intruder or incubus type experiences. People who had experienced episodes repeatedly, however, reported more floating types of hallucinations, which generally elicit less fear and anxiety. People who reported floating or out-of-body sleep paralysis episodes sometimes even found the experiences spiritual or enlightening more than frightening. 

Although some of her experiences with sleep paralysis were scary, says Susan Davis, a restaurant owner in Austin, Texas, the out-of-body hallucinations were merely strange. 

“I used to be able to float around the room and put my hand through the floor, feeling the layers of carpet, floor and ceiling of the floor below me,” she says.  

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2. And make you feel as though your breathing is restricted.

The main muscles that drive breathing are functioning during sleep, but you use additional muscles to breathe when you’re awake. During sleep paralysis, you’re not able to control those muscles as much as you are when you’re awake, which might make people feel they’re not getting enough breath during an episode, says Peterson. “It’s probably similar to stories you hear of people in surgery who are conscious and aware of what’s going on yet are paralyzed by anesthesia,” he says. “Sometimes they feel they can’t breathe even though they’re still getting adequate oxygenation.” 

3. Certain brain chemicals might be to blame.

University of Toronto researchers concluded in 2012 that blocking two nerve receptors in the brain might make it possible to paralyze the muscles that are active during sleep paralysis. 

But it doesn’t appear that that one study has spurred much additional research in this area. GABA, one of the neurotransmitters looked at in the study, is in most brain areas and involved in many different processes, Peterson says. 

“It’s not like some other neurotransmitters that you only interfere with a small number of brain areas,” he explains. “If you had a drug to block all GABA it would have broad and non-specific effects, so not a practical approach to understanding a process.”

4. Sleep disruption and anxiety can contribute to sleep paralysis. 

Changes in sleep routine, some medications and even nocturnal leg cramps might trigger sleep paralysis although scientists can’t yet pinpoint definitive causes. A study of twins concluded that anxiety, general sleep quality and “exposure to threatening events” all affected sleep paralysis susceptibility. They also noted a “moderate genetic influence” and called for more research of the possible effect of circadian rhythm disruption on sleep paralysis.  

The aforementioned amygdala tends to be overactive in people with anxiety or trauma-related disorders, Peterson notes, which might partly explain the link between anxiety and sleep paralysis episodes. 

5. It’s gaining traction as a possible explanation for supposed paranormal occurrences.

Sleep paralysis appears to have been part of cultural lore and legend for centuries. It’s referenced in medieval Persian texts, is thought to explain the scourge of the bat demon of Zanzibar, and is suspected to have inspired similar bogeyman type legends all over the world.

Some researchers suspect that accusations of witchcraft stemmed from episodes of sleep paralysis, and more recently, the authors of one study regarding the link between sleep paralysis and alien abductions concluded that at least half of their study participants who claimed they’d been abducted by aliens actually had experienced sleep paralysis. (Others, however, are skeptical of this skepticism.)

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6. There is no specific treatment for it.

Antidepressants are sometimes prescribed in severe cases of sleep paralysis but there’s no generally accepted treatment for it.

“There isn’t great evidence at this point for particular medications to treat or prevent it,” Peterson says. 

7. Improving sleep hygiene and mindfulness meditation and cognitive behavioral therapy techniques seem to help.

Prevention strategies are similar for people who suffer from different parasomnias: Maintain healthy routines such as regular sleeping and waking hours, avoiding disruptors such as alcohol and caffeine and for whatever reason, not sleeping on your on back. According to a 2014 study of frequent isolated sleep paralysis sufferers, of the 19 percent who tried to prevent episodes from occurring, almost 80 percent said they were successful. 

Disruption techniques were slightly less successful in that study, but many experts and sleep paralysis sufferers say they can help, too. 

“Apart from reducing those things that make sleep paralysis more likely, it helps to have some awareness about it,” Peterson says. “If you’re aware that you’re having these episodes, it can help make them less distressing and negative.” 

Some of the advice is similar for people who suffer from frequent nightmares. Cognitive sleep therapy techniques can help people exercise more control over the experiences of nightmares and sleep paralysis. Some say it can help to just take an inventory of what’s happening to you physically, such as noting tingling toes.  Concentrating on moving a finger, for example, can help a sleep-paralyzed person to regain a sense of control. 

Researchers are also exploring how mindfulness meditation techniques might help reduce the fear and anxiety of people during sleep paralysis episodes. A recent study suggests four steps: “reappraisal” of the perceived threat, emotional distancing, muscle relaxation and focusing attention inward. 

“The state of mindfulness, or being slightly apart from yourself and objectively looking at yourself, can help make those experiences less distressing when they’re happening,” Peterson says.