Demons — Explained by the Science of Sleep

It’s not demons. It’s hypnagogia and sleep paralysis.

Image by author. (CC BY-SA 4.0)

I never believed in demons, but that night… that night I had one sitting on my chest.

It was faceless.

It had a woman’s shape.

And it was pure evil.

My flight or fight response was yanked all the way to flight, my amygdala yelling at me “Flight! Flight! Flight! Get out! Now!”

But I couldn’t move. I couldn’t scream. I couldn’t fight. I was helpless.

I was paralyzed.

I could just lie there on my back, feeling the immense pressure of that thing, as it sat there, crushing down on me, suffocating me, with a black void where it’s face was supposed to be.

I knew what was going on. Still, I was scared shitless.

Even when you know what’s going on, becoming the victim of what seems to be a demonic attack is horrifying. If you’ve experienced it yourself, you know what I’m talking about. If not, imagine waking up at night, it’s dark around you, and both your voice chords and your body are paralyzed. Only your eyes are under your control. You sense a menacing presence. All of a sudden you feel a weight descending on your chest. You open your eyes, full of anxiety and dread, and then you see this scary creature or this one. And you still can’t move or scream! You’d be scared out of your mind, just as I was.

If these brutes have never tormented you, be glad. But don’t feel too safe. There is at least a 10% chance that you’ll have to live through such a frightful encounter at least once in your lifetime. Studies (here and here) have shown that among the general population, approximately 1 in 10 people have had these encounters. And among certain groups of people, in particular college students, it can even be as high as 4 out of 10 people.

Luckily, there’s a good scientific explanation for these abominations and a few things you can do to fight them off.

Demons are everywhere

Have you ever wondered where the word “nightmare” comes from? If you thought it has anything to do with female horses, you’re wrong. The “mare” in “nightmare” comes from the Old English “maere”, which is a female demon that rides on people’s chests while they sleep.

There’s similar evidence of such demons in other European languages.

The French word for nightmare, “cauchemar”, contains “cauche” and “mar”. “Mar” has the same meaning as the Old English “maere”. It refers to a female demon. “Cauche” derives from the Old French verb “chauchier”, which means “to press”. “Cauchemar” thus literally means “a demon that presses down on you”.

The Spanish word for nightmare, “pesadilla”, comes from “pesar”, which means “to be heavy”, and it contains the Spanish female diminutive suffix “-illa”. It thus literally means “the little heavy one”.

The German word for nightmare, “Albtraum”, is the composite of “Alb” and “Traum”. “Traum” means dream and “Alb” refers to an evil elf that sits on one’s chest while sleeping. You can see depictions of these vicious elves in some eighteenth century paintings:

18th-century painting by Johann Heinrich Füssli. Image in public domain.
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18th-century engraving by Thomas Burke. Image in public domain.

(If you’re wondering about the horses in these paintings, according to Wikipedia, they are probably “a visual pun on the word ‘nightmare’”.)

Early 20th-century drawing by Fritz Schwimbeck. Image in public domain.

When nighttime encounters with demons have a sexual component, medieval European mythology referred to them as incubi and succubi. That is, incubi and succubi are demonic sexual predators. The incubus is the male form and attacks women while they sleep. The succubus is the female counterpart that attacks sleeping men.

The terms incubus and succubus, however, are much older than medieval and originate with the Romans. And before the Romans, the attacks were attributed by the ancient Greeks to Ephialtes, a mythological giant and son of Poseidon, the god of the sea.

Non-European cultures also have stories of similar events. Here are a few examples:

These are just a few examples. There are plenty more and it’s no coincidence that all accounts share the stereotypical features of paralysis and pressure on the chest.

The scientific explanation

So what the hell is going on? Why are there tales all over the world of demons climbing on people’s chests after paralyzing them during their sleep? Why did one of these wretched beasts pounce on me? And what about you? Could you be the next victim? And is there anything you can do?

Let’s see.

The sleep cycle

Sleep can be broken down into two distinct phases: non-rapid eye movement sleep (NREM sleep) and rapid eye movement sleep (REM sleep).

During REM sleep, your eyes move around under your eyelids and you have your most vivid dreams. During NREM sleep, by contrast, your eyes are still and you dream less.

As you sleep through the night, you’ll cycle between NREM and REM sleep repeatedly, each such cycle being called a sleep cycle.

Take a look at the following hypnogram, which is a plot of the stages of sleep as a function of time:

Image by RazerM (CC BY-SA 3.0).

When you hit the pillow near midnight, you begin by drifting off into sleep. That’s stage 1 of NREM sleep. Then you descend further into light sleep, which is stage 2, and from there you plunge into deep sleep, which comprises stages 3 and 4. From deep sleep you then climb up to REM sleep. This completes one sleep cycle.

You then again descend into deeper stages of sleep to then once more return to REM sleep. This completes a second sleep cycle. As you continue to sleep, you go through further sleep cycles, each time descending through different stages of NREM sleep and then again climbing up to REM sleep.

Each sleep cycle takes between one and two hours and depending on how long you sleep you go through more or less cycles.

When it comes to demons, REM sleep holds the key to explaining the nighttime attacks.

How REM sleep explains demons

When the demons attack, it's always near REM sleep. We know that because REM sleep has some features that clearly distinguish it from NREM sleep:

  • During REM sleep you experience vivid dreams and
  • during REM sleep you undergo REM sleep atonia.

REM sleep atonia is a mechanism of your body that prevents you from acting out your dreams. Without it, any bedfellow of yours would wake up pummeled and bruised if you had a dream about a boxing match. More importantly, without this mechanism you yourself might not look much better after falling off your bed.

So how does REM sleep atonia work?

Your muscles are under your voluntary control thanks to motor neurons. These motor neurons are like cables that allow you to send signals from your brain to your muscles to make them move. REM sleep atonia works by inhibiting these motor neurons. As a result, when you’re under REM sleep atonia and your brain tries to send a signal to make your body move, the signal simply fizzles out before reaching its destination. The result? You’re paralyzed.

REM sleep atonia is absolutely normal and happens to you every night. Usually you’re not aware of it. You just peacefully snore through the night and then in the morning, when you wake up, you might remember a dream, but you don’t realize that your body had been paralyzed.

Sometimes, however, the part of your brain that makes you conscious awakens while the rest continues in REM sleep. This is so because your brain doesn’t transition from wakefulness to sleep and vice versa all at once. Instead, it’s a synchronized process between several parts of your brain. And sometimes that synchronization fails and one part of your brain lags behind another when you wake up. When that happens, you can enter a freaky in-between state known as hypnagogia. As the neurologist Dr. Steven Novella put it in his 2018 book The Skeptics’ Guide to the Universe,

Hypnagogia is a neurological phenomenon that can occur when a person is waking up (hypnopompic) or going to sleep (hypnagogic). It’s an in-between state where the person is neither fully awake nor fully asleep. In this state, very realistic images and sounds can be experienced. Although visual and auditory hallucinations are most common, experiences can range from hearing your name whispered to ones involving all the senses, including touch. They are, in essence, dream experiences that are occurring while you’re semi-awake.

Hypnagogia itself is not necessarily unpleasant. It’s just plain weird. For instance, I once had a false awakening where I got out of my bed, walked a few steps, turned around, and then saw myself still lying there asleep — an experience called an autoscopic hallucination.

Hypnagogia turns from weird to hell when you’re not only dreaming while awake, but your body continues to be under REM sleep atonia. When that happens, you’re experiencing sleep paralysis — you’re conscious but your motor neurons have not been reactivated yet.

Now you can imagine how that plays out.

You become conscious and realize that you’re paralyzed. You try to move, but you can’t. Shit, what’s going on? Fear starts to creep in and that, coupled with your hallucinatory state of mind, makes a horrid combination. You begin to see things that are not there. And not just anything, but things you’re programmed by evolution to fear the most: smart evil predators that are looking for one thing — to cause you harm.

Your hypnagogic trip turns into a vivid nightmare.

Why the demon sits on your chest

Hypnagogia and sleep paralysis explains the frightening hallucinations and the inability to move. But what about that feeling of pressure on the chest everyone’s talking about? Why does that demon always have to be a dick and sit on people’s chests? Where does that come from?

Well, during REM atonia your brain stem not only inhibits muscle control, but it also inhibits the voluntary control of your lungs. Now imagine how that would feel if you’re hallucinating the presence of the most evil creature your mind is capable of conjuring up. The fear makes you want to gasp for air, but you can’t. You have no control over your breathing. It continues as shallow as in normal REM sleep. You’re holy-crap-I-need-to-get-out-of-here-or-I’ll-die anxious and yet your breathing continues at its absolute minimum level. You’re struggling for air and that struggle utterly confuses your brain, turning that whole mess of a situation into the perception of something exerting pressure on your chest. Couple that with your hallucinations and this feeling of pressure feeds back into your nightmare. Now you’re full on hallucinating that most evil demon sitting right on top of you, on your chest, suffocating you with its weight.

How to survive a demonic attack

Okay, I’m being overly dramatic. You’ll survive whatever you do. Although unsettling, hypnagogia and sleep paralysis are harmless in the end. Still, there are a few things you can do to make it less likely to occur and at least two things you can do to help you snap out of it when it does occur.

First, you should know that poor sleep habits and sleep disruption makes it more likely for you to suffer from sleep paralysis. So the first defense is to have good sleep habits: go to bed and wake up at consistent times, avoid caffeine and alcohol before sleep, don’t play around with your phone or watch TV in your bed, minimize stress in your daily life, etc.

Second, it has been shown that sleep paralysis is more common when you’re sleeping on your back. So consider sleeping in a different position.

Third, learn about scientific skepticism and how the world actually works as it has been shown that superstition and the belief in the paranormal correlates with sleep paralysis. Of course it’s only a correlation and maybe people who suffer from sleep paralysis are just more likely to believe in the paranormal. Still, scientific skepticism never hurts.

As to what to do if you find yourself under sleep paralysis, here are two tips from people who have suffered from recurrent sleep paralysis that may work for you as well. The first comes from a user named mauxly on reddit:

Next time it happens, turn your fear into rage […] I have chronic sleep paralysis [and] I’ve tried lots of things with varying degrees of success, but straight up raging at [the meanies], even if you can’t move, gets them to go away.

Basically, yank your fight or flight response towards fight. Reddit is not the most reliable source of information, but this tip seems to help.

Here’s the other tip, from a user named purpledirt:

I don’t tend to get auditory hallucination, so keeping my eyes closed pretty well negates the entire experience (except for the actual feeling of paralysis). [Also,] I usually have minor motor control of my toes, and am able to break myself out of the paralyzed state by flicking my toes together, which creates enough stimulation to break through the paralysis and wake me up.

If neither tip works for you, just know that you’ll eventually wake up. End the end, this knowledge, together with good sleep habits, is what has helped me the most.

Wrapping up

Our brains have glitches and our perception and memories are deeply flawed.

Sleep is particularly weird and your brain can mess with you in spooky ways. One of these ways is a screwed up situation called hypnagogia, where you are half asleep and half awake. In that state, you’re conscious, but still dreaming. In other words, you’re hallucinating. Couple this situation with another part of your brain keeping your body immobilized to prevent you from acting out your dreams — even though you are no longer asleep — and you get sleep paralysis. Hallucinating and being paralyzed don’t make a pleasant combination and your mind, which is trying to make sense of it all, conjures up the most frightful devils.

Luckily, the situation is harmless and knowing that is key. Hypnagogia and sleep paralysis are nothing but a temporary brain glitch. If you ever suffer an episode accompanied by full-blown hypnagogic hallucinations, understanding what’s going on should make the experience just a little bit less frightening. At the very least, you’ll be able to make sense of it after it’s over and not fall prey to superstitious beliefs.

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Science fan, cartoonist, PhD, eukaryote. Doesn't eat cats, dogs, nor other animals.

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