REM AtoniaREM atonia is the scientific name for the phenomenon of sleep paralysis, in which your brain shuts off stimulation to your large muscle groups during Rapid Eye Movement, immobilizing you so you don’t act out your dreams. The natural process of REM atonia likely evolved to prevent us from injuring ourselves or wandering into danger during the dream state; other mammals and birds that have been studied while sleeping also exhibit protective REM atonia. However, this totally natural process can sometimes lead to a phenomenon called Aware Sleep Paralysis (ASP), in which you become conscious while your body is still immobilized. While researchers believe that conscious sleep paralysis is usually harmless (and may be quite common), it can be a very scary experience for people who don’t know what’s happening or how to stop it.

In humans, REM sleep comes in two stages: tonic and phasic. During the phasic stage, a sleeper might exhibit some small movements in their toes and fingers, and their limbs might twitch. In the tonic stage, muscle tone is almost totally absent in the limbs and body. Surprisingly, considering that sleep paralysis is such an important part of healthy REM sleep, scientists still aren’t sure of the mechanism by which the brain induces REM atonia: they do know that when you’re dreaming, the brain greatly reduces production of the neurotransmitters serotonin, norepinephrine, and histamine, and it’s possible that the reduced levels of these neurotransmitters causes paralysis of large motor muscles. In REM atonia the motor neurons of the brain aren’t stimulated; the result is a selective paralysis that lets your autonomic functions (breathing and heart rate) continue while preventing you from acting out your dreams.

People experience problems with sleep paralysis when they either come awake while their bodies are still in REM atonia (the more common form), or when their bodies enter REM atonia before they fall asleep. In this way, sleep paralysis could be thought of as an arousal disorder, though classing it as a disorder is problematic because it’s so prevalent: the latest estimates suggest that while it is most common in teenagers, up to 50% of people will experience sleep paralysis at least once!

If you’re not sure if you’ve ever experienced waking sleep paralysis/REM atonia, here are some things to ask yourself: have you ever woken and been unable to move, or felt like a great weight was pressing on your chest? The most common symptom of waking REM atonia is the inability to move major muscles (arms, legs, and body, though you may be able to wiggle fingers and toes). Sleep paralysis is often accompanied by vivid hypnagogic sensations, including hearing noises or voices; experiencing odors or tastes; feelings of levitation or floating; strong feelings of euphoria, terror, or orgasmic sensations; and seeing people or entities that aren’t there. Feeling that something or someone is in the room with you is one of the most common but least desirable symptoms of waking REM atonia: people who’ve experienced sleep paralysis often report that they encounter malevolent entities that threaten them or even seem to touch them as they lie unable to move! The frightening nature of these nocturnal visitors has led cultures worldwide to come up with supernatural explanations for sleep paralysis that involve being sat on or assaulted by ghosts or spirits. For instance, in China you have the gui ya, or “ghost pressure”, thought to be caused by a ghost that came and sat on the dreamer’s chest. In Japan there is the kanashibari, in Newfoundland the “old hag”, and the West Indies have a similar spirit called the kokma, or “ghost baby”.

In most cases, aware sleep paralysis is not the sign of an underlying medical condition. However, in some people it may be an early symptom of narcolepsy, a sleep disorder in which a person erratically enters REM sleep during their waking hours. Sleep paralysis is most likely to be a sign of narcolepsy if it is frequent and occurs mostly when the person is falling asleep rather than waking up. Some doctors have also postulated that people who suffer from untreated sleep apnea (in which your breathing is interrupted during sleep) are more likely to experience waking sleep paralysis; this is backed up by some anecdotal evidence from people whose sleep paralysis episodes stopped or greatly diminished after they got treated for sleep apnea.

If you experience frequent and unwanted sleep paralysis episodes, it’s important to get checked out by a doctor to rule out these and other possible medical conditions. The most common contributing factor for waking REM atonia in healthy people is disordered sleep patterns: you’re more at risk for sleep paralysis if you work a night shift and sleep during the day, have just been on a long plane ride (due to jet lag), or if you become chronically sleep deprived. This is why people who are trying to induce sleep paralysis to generate an out-of-body experience will often deliberately disrupt their sleep patterns. Establishing good sleep hygiene by sleeping in a quiet, dark room and turning off the TV and computer an hour or so before bed will really help you achieve deeper sleep with less likelihood of a sleep paralysis episode.

You can also reduce your incidence of sleep paralysis with regular exercise, especially qigong and yoga, by maintaining a healthy diet, and through meditation. Some sleeping medications can occasionally cause sleep paralysis as a side effect, so check with your doctor if you’re taking anything to help you sleep. Finally, although it can be a distressing experience, people have also discovered that they can put their sleep paralysis episodes to use as gateways to a lucid dream or out-of-body experience. For instance, sleep researcher Ryan Hurd’s Sleep Paralysis: A Dreamer’s Guide lays out mind-training techniques you can use to confront and banish malevolent entities and turn your unpleasant sleep paralysis episodes into exciting lucid dreams!