What is sleep paralysis, and why do people sometimes sense a presence in the dark?

You wake up in the dark. You are fully aware, but you cannot move or speak. Then you feel it: someone is in the room. Maybe a weight presses on your chest. Soon it is over, and your heart is pounding. If this has happened to you, you are far from alone, and there is a name for it.

It is called sleep paralysis. It happens as you fall asleep or wake up. Your mind is awake, but your body is still in the mode of REM sleep, the stage of sleep when vivid dreams are most common. During REM sleep, your muscles normally don’t move. MedlinePlus, from the U.S. National Library of Medicine, says an episode lasts from a few seconds to a minute or two; Britain’s National Health Service (NHS) says the feelings can last up to several minutes. A 2011 review of 35 studies found that about 7.6% of people in the general population have had at least one.

The sense of a presence is one of its common features. A research team at the University of Waterloo in Canada found that a sensed presence, fear, and strange sounds or shapes tend to come together, as do chest pressure and trouble breathing. The team noted that such accounts are remarkably similar across time and cultures. People have explained them as witchcraft, evil spirits and even aliens. In one Cairo study, nearly half (48%) of people from the general population with sleep paralysis blamed the jinn, spirit-like beings from Islamic tradition.

Why would a waking brain feel someone who is not there? In the lab, they have made healthy people feel a presence by scrambling the brain’s signals about the person’s own body. When it is hard to see, people also tend to assume that others are looking at them. Darkness itself makes us jumpier.

Sleep paralysis is real, common and well studied. The NHS calls it harmless, though it can be scary. The presence feels real because the experience is real. The evidence points to the half-awake brain as its source. We label this Established.