It was gone, but I was still screaming. Its form is hard to describe now, thinking back to the night when I woke up terrified in the dark. I remember some sort of floating orb with a red, ghoulish face surrounded by black smoke. I can’t help but describe my demon as an evil-looking Gastly. Right then and there, as I leaned up in bed, I understood why peoples across culture and time have said they’ve been visited by demons in the night. It would have made a believer out of me too, had I not known about the visceral, unforgettable experience of sleep paralysis.

If you’re sleeping well, the brain and the body usually get along. After drifting off to sleep, the brain eventually reaches the only stage we can remember: the REM (rapid eye movement) stage, characterized by rapid movement of the eyes underneath their lids. Here we dream, and just to make sure those dreams aren’t aped by our bodies, the brain releases two chemicals, a neurotransmitter called glycine and a nerve receptor in muscles called GABA, that paralyze the muscles we can move voluntarily.

During normal sleep, these chemicals wear off by the time we wake. But if you’re sleep deprived, on certain medications, or simply unlucky, the paralyzing chemicals will still be active as the brain wakes in the middle of REM sleep. You become conscious while dreaming, unable to move. When this happens, people report seeing everything from ghosts to demons to mysterious black figures on or near their beds. Sleep paralysis strikes an estimated 6.2 percent of the general population at least once in their lives.

For whatever reason, despite the wonderlands of imagination dreams transport us to, the experience of sleep paralysis has surprisingly common themes. There is usually an overcoming sense of fear and dread accompanied by the vision of some dark, humanoid figure on or around the bed. Breathing is also reportedly difficult, with a feeling of choking or pressure on the chest. This last symptom is common enough that cultures have made it the demons’ defining quality. Indonesian suffers of sleep paralysis call it “digeunton,” meaning “pressed on.” In Hungarian it is “boszorkany-nyomas,” or “witches’ pressure.” In Turkey it is “Karabasan,” in Thailand it is a ghost of the “Phi Am” folklore, and in the southern United States it is “witch riding.”

Rodney Ascher’s The Nightmare | Campfire

Given how terrifying sleep paralysis can be, the supernatural explanations are understandable. There simply isn’t another apparently conscious experience like it. While many people who have had sleep paralysis will only experience it once or twice in a lifetime (for others with chronic conditions like narcolepsy it can occur much more frequently), the event is debilitating. A demon stepped out of the ether and into your bedroom. It doesn’t matter if it won’t again; knowing that the portal is open and what can come through is enough.

But now that we know many if not all of the symptoms of these encounters can be attributed to brain chemistry and out-of-sync sleep stages, the supernatural explanations become much less likely. The fact that many people of different cultures and time periods report a choking, disturbing figure at the end or side of a bed could very well be a description of a ghost or demon that haunts people in the same way. It could also be that, given many people have the same mental picture of what a demon should look like and similar brain chemistry, sleep demons are a quirk of common sleep conditions.

The distinction between the natural and supernatural here isn’t without consequence. By definition, the supernatural is beyond control. The natural is not. With proper sleep hygiene, therapy, and sometimes medication, we can make the demons disappear.

After I saw my demon, I let out a scream I didn’t know I had in me. The vision had appeared at the edge of my bed, almost inscrutable, and vanished as consciousness fully returned. In that moment, I realized that had I not known of sleep paralysis, I would have become paralyzed when awake. The fear would have trapped me in a prison of circumstance; I would always know that I might wake up with terror literally sitting on my chest. But I also knew that our knowledge of the brain has evolved past the point where hallucinations, even common ones, can rule in the real world. The demons haven’t bothered me since.

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Featured image: “John Henry Fuseli – The Nightmare” by Henry Fuseli. Licensed under Public Domain via Commons

Kyle Hill is the science editor at Nerdist Industries. Follow on Twitter @Sci_Phile.