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I don't dare sleep on my back any more. In that position, I often wake up from dreams only halfway – that is, my mind wakes up, but my body remains immobile. I can still think, still sense sunlight trickling through the curtains, still hear passersby on the street below. But when I try to move, nothing happens. I can't even twiddle a toe or flex a nostril. It's what being reincarnated as a statue would feel like. It's the opposite of sleepwalking: It's sleep paralysis.

The worst part is the terror. My mind, fully awake, expects my lungs to take full gulps of oxygen. But my body, still asleep, takes mere sips of air. I feel I'm suffocating, bit by bit, and panic starts smouldering in my chest.

As bad as that sounds, many sleep paralytics have it worse. My episodes last a few minutes at most, but some people's drag on for hours. And while I can be shaken awake, some people can't. One poor woman in England has been declared dead three times, and once woke up in a morgue. Other people have out-of-body experiences, and the unluckiest ones of all feel an evil "presence" – a witch, demon, alien, or other monster – pressing down on their bodies, smothering them.

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Sleep paralysis doesn't actually open a portal into the supernatural, of course. And it doesn't offer proof of dualism, either: The mind cannot function outside the body, independent of it. To the contrary, sleep paralysis is a natural byproduct of how our brains work.

Deep inside the lower brain, in the brain stem, sits an inch-long hump called the pons. After we fall asleep, the pons initiates dreaming (rapid eye movement (REM) sleep) by sending chemical signals to higher parts of the brain, where dreams stir to life. During dreams, the pons also tickles the spinal cord just beneath it, which produces chemicals to make your muscles flaccid. This temporary paralysis prevents you from acting out nightmares by fleeing the bedroom or taking swings at werewolves.

While mostly protective, this paralysis can backfire. Sleeping on your back can collapse the airways in your throat and deprive your lungs of oxygen. This isn't a huge deal during non-dream sleep: Your brain senses this, your mind rouses you halfway awake, and your body shifts posture or rolls over. To shift your body during REM sleep, though, the brain also has to order the pons to stop paralyzing your muscles. And for whatever reason – a chemical imbalance, frayed neurons, pure mulishness – it doesn't always obey. So while the brain succeeds in rousing the mind, the muscles remain limp.

Things go south from there. If this limbo persists, the mind senses something is amiss and panics. And again, some people have things much worse. At least with me, the actual dream I'm having stops as soon as my mind wakes up. Not so in other people: They never quite escape the dream state. They're alert to their surroundings, they're paralyzed, and their brains keep conjuring up nonsense. That's probably why they blame the paralysis they feel on demons and aliens and the like: In their minds, the dream hasn't ceased, and it transforms into a real, living nightmare.

So, yeah, there's a reason I stopped sleeping on my back. But even though I dread sleep paralysis, it did teach me something valuable. We tend to divide the brain into "higher" and "lower" parts, but it's not so simple. All parts of the brain work together to produce consciousness, and even a tiny flaw in the brain stem can intrude on the most rarefied realms of the human mind.

Sam Kean is the author of The Disappearing Spoon and The Violinist's Thumb. He's currently working on a new book about neuroscience.

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