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Brian Sharpless woke up in the middle of the night with the haunting feeling that he wasn’t alone. Slowly, his bedroom door creaked open, like a hand was lightly touching it. Moonlight from the hallway window streamed through the open door. He wondered if he was getting robbed. Then Sharpless discovered something even more alarming: He couldn’t move. He lay paralyzed as he watched a dark form with a long, serpentine neck creep through the doorway. “I saw a shadowy face that looked like a ninja mask with red eyes,” Sharpless told me. It stared at him, moving closer and closer, and Sharpless felt … relieved.

Sharpless is a researcher who studies sleep disorders, and as he watched this demonic creature slip into his bedroom, he realized he wasn’t in danger. He was, for the first time, experiencing sleep paralysis himself.

Normally while you’re dreaming, your body is paralyzed so that you don’t act out your dreams. But in sleep paralysis, you wake up while your body is still frozen. And even though you are fully conscious, looking around your real room, part of your brain is still dreaming. People often hallucinate creepy things — shadowy forms, monsters, and aliens — in their bedrooms.

I talked to Sharpless, associate professor at the American School of Professional Psychology and author of Unusual and Rare Psychological Disorders, and Michael Breus, fellow of the American Academy of Sleep Medicine and founder of The Sleep Doctor website, to learn more about this mysterious disorder and what you should do if you find yourself in the middle of it.

How common is sleep paralysis?

As weird as waking up to hallucinations sounds, the experts I talked to say it’s more common than you might think. An estimated 8 percent of the worldwide population has experienced it, according to Sharpless, and that’s a conservative estimate. The number jumps to 28 percent for college students and 32 percent for psychiatric patients.

“I think really anybody could have it if the circumstances are right,” Sharpless said. Just being sleep deprived can make someone experience it. In fact, researchers induce sleep paralysis in labs all the time. They wait until a person has fallen asleep and gone into REM, then touch them lightly. Sometimes, that’s all it takes.

Even though many haven’t heard of the term “sleep paralysis,” people knew about the disorder long before there were sleep labs or even modern countries. Ancient Greeks called it “Pan ephialtes,” explained Sharpless, which means something like “when the god Pan leaps on your chest.” In fact, the word “nightmare” used to mean to what we now call sleep paralysis. In Middle English, nightmare referred to a “female spirit or monster” that would “settle on and produce a feeling of suffocation in a sleeping person or animal,” according to the Oxford English Dictionary. (“Mare” used to mean “evil female monster.”) After all, people dealing with sleep paralysis often feel as if a monster — be it a Greek god, a female spirit, or an alien — is sitting on their chests. “Nightmare” only started meaning “bad dream” in the 20th century.

What causes sleep paralysis?

Sleep is, to drastically understate matters, really complicated. For one thing, the different states of sleep — light dreaming, deep dreaming, etc. — aren’t actually so separate. A lot can go on in the transition between being awake and being asleep.

“The more complicated you make a computer or the more complicated you make a car, the more things can go wrong,” Sharpless explained. In sleep paralysis, you wake up before your brain stem, which stops your muscles from moving while you’re asleep, realizes it’s awake. As a result, it continues sending neurotransmitters that tell your muscles to freeze. Scientists still don’t know why this happens.

How do these hallucinations work?

“The hallucinations seem to happen in stages,” explained Sharpless. First, you wake up with the sense that you’re not alone. “It’s kind of like when you’re walking down a lonely street at night,” Sharpless continued. “You wake up, your eyes open, and you feel this uneasy feeling.”

As your brain tries to make sense of what’s going on, it creates something to explain why you feel so uneasy. It might whip up a menacing figure to watch you, often one that fits into your worldview.

“If you’re in 21st century America, you might see an extraterrestrial,” Sharpless said. “If you were in 15th century France, you might see a witch or a demon.”

It doesn’t stop at seeing imaginary things. You can feel them too. Sharpless says that, just as the brain can create imaginary sights and sounds, it can create imaginary physical sensations, too. People say that the monster in their room sits on them, and they can sense the pressure on their chest. Sharpless notes that the anxiety from seeing monsters may create or add to this feeling, since anxiety can cause chest pain. Sometimes, people experiencing sleep paralysis can’t breathe for a few seconds. In really scary cases, people feel like something is attacking or sexually assaulting them.

Are some people more likely to have sleep paralysis than others?

Sharpless says that anyone who easily dissociates — that is, goes into a kind of trance where they lose awareness of the world around them — may be more likely to have sleep paralysis. That might conjure images of prophets or fortunetellers, but dissociation is usually pretty mundane. If you drive along a highway and you zone out, then come to after a couple miles, you’re dissociating. People who do so more often might be more likely to develop sleep paralysis.

How do you stop sleep paralysis when it happens to you?

Nobody actually knows how to stop sleep paralysis, but there are ways to make it less scary. Simply knowing what’s going on can help. During Sharpless’s own sleep-paralysis episode, just knowing he was hallucinating made “the demonic thing with the giraffe neck” seem silly rather than scary, he explained.

“Either look at it dismissively, ignore it, or find humor in it,” he suggested. “You might say to yourself, ‘“Am I really seeing a demon in the bedroom now? Really?’” You can also try and focus on a body part or reassure yourself that what you’re seeing isn’t real. If your brain is creating an imaginary monster, making your brain pay attention to something else can cause the imaginary thing to cease to exist, or at least not be so scary. Sharpless says some of his patients have tried these methods and reported that they help, as they takes focus off of the monsters.

Is sleep paralysis harmful to your health? Can you die from it?

“Other than that it scares the crap out of you, it’s not dangerous,” Breus told me. Still, your reaction to sleep paralysis could cause problems. Some people who get recurring sleep paralysis take stimulants at night or try other methods to stay awake, which can be pretty unhealthy. In one 19th-century case, a man paid his manservant to watch him all night in order to wake him up if he got sleep paralysis. (Spoiler: This did not help him sleep.)

And then of course, if you don’t know what’s going on, you could believe all sorts of creepy things when you’re awake. You might think you’re being regularly attacked by demons or kidnapped by aliens. “I’ve had people say to me, ‘the Devil possessed me,’” Breus said.

These sorts of beliefs can really strain your relationships with friends and family. “Some people think they’re going crazy,” said Sharpless. It’s especially tough on people who already have psychosis; they have an even more difficult time distinguishing between reality and hallucination.

How do you treat sleep paralysis?

If you only get sleep paralysis occasionally, you don’t need to treat it. But some people get it every day, or even a few times a night. They may choose to go on antidepressants to help them sleep better. Working on underlying issues like anxiety can help too.

How do you prevent sleep paralysis?

There are some simple things you can do. Sleeping on your side makes sleep paralysis less likely. Avoiding alcohol improves matters since alcohol interferes with sleep. Most importantly, getting regular, quality sleep is the best way to avoid sleep paralysis. Sharpless, for instance, got his moment of sleep paralysis when he was jetlagged from giving a presentation in England on, coincidentally, sleep paralysis.

“Now I’m wondering if I’m going to get sleep paralysis or something,” I told Sharpless after we’d been talking about the disorder long enough to make me nervous.

“You just might,” he said playfully. “They used to talk about it being contagious in ancient Rome, but we don’t have any good data on that.”