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In normal sleepers, muscle control resumes before they become consciously awake.
But a mistimed disconnection between the brain and body can result in temporary paralysis while the person is mentally aware.
Your heartbeat may increase during this stage and muscles may twitch occasionally.
During REM sleep, muscles that move the eyes, and those involved in breathing, continue to function, but most of the body's other muscles are stopped.
To begin to understand how these disorders have such frightening effects, it's important to know how the sleep cycle works.
Sleep can be divided into four stages during which your brain activity, heart rate and breathing rate slow down.
The combination of dream and consciousness when awake can cause terrifying hallucinations.
Narcolepsy also includes features of dreaming while awake.
The earliest Chinese dictionary, the Shou Wen Chieh Tzu, dating from the Eastern Han dynasty around AD30 to AD124, records for the first time the Chinese written character for ghost oppression, which combines the characters for oppression and ghost.
Sleep paralysis affects fewer than 8 per cent of the general population, although about 25 per cent of all people have probably experienced it at some point during their life.
Interestingly, interviews with people who have had an out-of-body or a near-death experience, or both, indicate that the two phenomena are similar.
Dr Kevin Nelson from the University of Kentucky believes the brain's arousal system that regulates different states of consciousness such as REM sleep and wakefulness may be the cause of out-of-body displays.
Sleep paralysis is a form of intrusion into REM sleep.