Imagine this: You're fast asleep when all of a sudden you're awoken! And not by your alarm clock. Your eyes open, and there's a demon sitting on your chest, pinning you down. You try to open your mouth and scream, but no sound comes out. You try to get up and run away, but you realize that you are completely immobilized. The demon is trying to suffocate you, but you can't fight back. You've awoken into your dream, and it's a nightmare. It sounds like a Stephen King movie, but it's actually a medical condition called sleep paralysis, and about half of the population has experienced this strange phenomenon at least once in their life. This panic-inducing episode of coming face-to-face with the creatures from your nightmares can last anywhere from seconds to minutes and may involve visual or auditory hallucinations of an evil spirit or an out-of-body feeling like you're floating. Some have even mistaken sleep paralysis for an encounter with a ghost or an alien abduction. In 1867, Dr. Silas Weir Mitchell was the first medical professional to study sleep paralysis. "The subject awakes to consciousness of his environment but is incapable of moving a muscle. Lying to all appearance, still asleep. He's really engaged for a struggle for movement, fraught with acute mental distress. Could he but manage to stir, the spell would vanish instantly." Even though Dr. Mitchell was the first to observe patients in a state of sleep paralysis, it's so common that nearly every culture throughout time has had some kind of paranormal explanation for it. In medieval Europe, you might think that an incubus, a sex-hungry demon in male form, visited you in the night. In Scandinavia, the mare, a damned woman, is responsible for visiting sleepers and sitting on their rib cages. In Turkey, a jinn holds you down and tries to strangle you. In Thailand, Phi Am bruises you while you sleep. In the southern United States, the hag comes for you. In Mexico, you could blame subirse el muerto, the dead person, on you. In Greece, Mora sits upon your chest and tries to asphyxiate you. In Nepal, Khyaak the ghost resides under the staircase. It may be easier to blame sleep paralysis on evil spirits because what's actually happening in your brain is much harder to explain. Modern scientists believe that sleep paralysis is caused by an abnormal overlap of the REM, rapid eye movement, and waking stages of sleep. During a normal REM cycle, you're experiencing a number of sensory stimuli in the form of a dream, and your brain is unconscious and fully asleep. During your dream, special neurotransmitters are released, which paralyze almost all of your muscles. That's called REM atonia. It's what keeps you from running in your bed when you're being chased in your dreams. During an episode of sleep paralysis, you're experiencing normal components of REM. You're dreaming and your muscles are paralyzed, only your brain is conscious and wide awake. This is what causes you to imagine that you're having an encounter with a menacing presence. So this explains the hallucinations, but what about the feelings of panic, strangling, choking, chest pressure that so many people describe? Well during REM, the function that keeps you from acting out your dreams, REM atonia, also removes voluntary control of your breathing. Your breath becomes more shallow and rapid. You take in more carbon dioxide and experience a small blockage of your airway. During a sleep paralysis episode, a combination of your body's fear response to a perceived attack by an evil creature and your brain being wide awake while your body is in an REM sleep state triggers a response for you to take in more oxygen. That makes you gasp for air, but you can't because REM atonia has removed control of your breath. This struggle for air while your body sleeps creates a perceived sensation of pressure on the chest or suffocation. While a few people experience sleep paralysis regularly and it may be linked to sleep disorders such as narcolepsy, many who experience an episode of sleep paralysis do so infrequently, perhaps only once in a lifetime. So you can rest easy, knowing that an evil entity is not trying to haunt, possess, strangle, or suffocate you. Save that for the horror films!