Why are demons blamed for sleep paralysis?

An incubus squats atop a dreamer's chest in Henry Fuseli's iconic 1781 painting "The Nightmare."
An incubus squats atop a dreamer's chest in Henry Fuseli's iconic 1781 painting "The Nightmare."
Fine Art Images/Heritage Images/Getty

In "The Book of Imaginary Beings," Jorge Luis Borges describes a Chinese myth in which reality and the world beyond the mirror are separated by an uneasy truce. When that truce inevitably breaks, the strange denizens of the specular world will spill back into our own -- and a gleaming fish of unnatural color will be the first to break through the looking glass.

The experience of sleep paralysis is very much like a glimpse of that portentous fish. It distorts the line between the world of unconscious dreaming and our conscious experience of reality. Because, indeed, there's a biological truce between sleep and wakefulness.

And when it breaks? That's when the demons creep in.

Our most vividly remembered dreams occur during the depths of rapid-eye movement (REM) sleep. Bland re-enactments of daily life, surreal fantasies, erotic trysts and unspeakable horrors -- it all goes down in this unconscious shadow realm. And since dreaming is, in essence, a mental simulation, the brain puts the physical body on a kind of lockdown. Except for shallow breathing and eye movements, the dreamer's body persists in a state of safe paralysis.

That's the plan, anyway: The demons stay in their unconscious cage, safely removed from our conscious reality. But sometimes this safety feature of REM sleep malfunctions. The brain wakes up, but the body remains paralyzed in "safe mode."

Imagine waking up in such a state, either shortly after falling asleep (hypnagogic sleep paralysis) or in the twilight stillness of the morning (hypnopompic sleep paralysis): You can't move. You can't talk. You may feel the weight of some alien body pressing down on your chest -- or even kinesthetic sensations, such as feelings of being dragged from your bed, vibrating, flying or falling.

And then there are the hallucinations -- the true, terrifying colors of Borges' mirror fish ripped from the world of dream into our own. Because both the hypnagogic (falling asleep) and hypnopompic (waking) states are highly susceptible to hallucination. In the former, the descending rational mind tries to make sense of nonlinear dream images. In the latter, the emerging dream-mind tries to make sense of real-world sights and sounds in the surrounding environment.

The hypnopompic state is often accompanied by vivid, lingering imagery -- and it's the stuff of dreams, so the dreamer's sexual fantasies, belief system and pop culture are likely to color the visions and sensations ripped from the dream world.

Finally, imagine all of this hallucination and bodily sensation wrapped around a strong sense of presence -- the unmistakable and primitive sensation that a menacing being or entity has invaded your space.

Now you might wonder why such an encounter has to be malevolent. Why don't those dreams of Jon Hamm or Scarlett Johansson snuggle fests ever follow us into the experience of sleep paralysis? It's largely a combination of situational and individual factors.

The experience itself is typically one of paralysis, labored breathing and an inability to speak -- attributes rarely associated with a comforting embrace. But then the person's cognitive style, distress sensitivity and supernatural beliefs may exasperate the experience's negative connotations [source: Cheyne and Pennycook]. But to be sure, not every sleep paralysis experience is traumatizing. People with a more analytical worldview tend to experience less post-episode distress, and those who repeatedly experience sleep paralysis sometimes come to roll with the experience, or transcend it into a more lucid, positive encounter.

I've asked you to imagine these things, but perhaps you don't need to. Depending on who's serving the stats, between a third and a full half of the general population has experienced sleep paralysis [source: Sacks]. The exact cause and psychological process of sleep paralysis remain somewhat elusive, but studies confirm that attacks are particularly likely if the sleeper enters REM sleep quickly after hitting the pillow, bypassing the stages of non-REM sleep that usually happen first. Stress and sleep pattern disruption also can affect the chances of such seemingly unnatural visitation.

It should come as no surprise, then, that accounts and mythologies of malevolent sleep visitations permeate every human culture.

On the next page, we'll ponder the demonic explanations.