Have you ever woken up at night and found that you could not move? You may also feel that someone is in the room. This is called sleep paralysis. It is not a ghost.

When we dream, the brain stops the body from moving. Sometimes the mind wakes up before this stop is turned off. It usually ends in a few minutes. It happens more often when people do not sleep enough.

Waking up at night unable to move, while feeling that someone is in the room, can be terrifying. Many people have thought it was the work of ghosts, but it is actually a well-understood condition called sleep paralysis. During REM sleep, when we have vivid dreams, the brain locks most of the body's muscles so that we do not act out our dreams.

Usually, this lock is released as soon as we wake up. Sometimes, however, the mind wakes up first while the body stays locked for a few seconds or minutes. Because the brain is still partly dreaming, people may see shadows or feel pressure on their chest. A 2011 review estimated that about 8 percent of people and about 28 percent of students experience it.

Waking in the night unable to move, often with a sense of an intruder or a weight on the chest, has been attributed to supernatural forces in many cultures. Scientists, however, explain sleep paralysis as a mismatch between brain states. During REM sleep, the period associated with vivid dreaming, most skeletal muscles are temporarily switched off, a state known as atonia, which prevents us from physically acting out our dreams. Normally this atonia ends the moment we wake, but occasionally consciousness returns while the muscles remain inactive for seconds or minutes.

Because dream-like activity can persist, people may perceive figures, footsteps or pressure. A 2011 systematic review of 35 studies involving roughly 36,000 people estimated lifetime prevalence at about 8 percent in the general population, 28 percent among students and 32 percent among psychiatric patients. A 2018 review linked the experience to poor sleep quality, irregular schedules and stress. These figures should be treated as approximate, though, because estimates varied widely depending on how researchers defined the condition and worded their questions, and most of the associated factors come from cross-sectional surveys.