"It is the complete opposite, actually," Breus explains. When you enter deep REM sleep, your brain tells the body's voluntary muscles to relax and go into almost a state of paralysis, which is called atonia. Atonia actually helps protect the body from injury by preventing you from acting out the physical movements in your dreams. In other parasomnias, such as sleepwalking or REM sleep behavior disorder, atonia does not occur properly and the voluntary muscles move while the mind remains asleep, which is why people can sometimes do crazy things in their sleep and be totally unaware of it.
In sleep paralysis, the body remains paralyzed in REM atonia while the brain awakens and the eyes start to open, explains Breus. Sufferers become alert in a transient conscious state, but they are unable to move voluntary muscles or speak. Although involuntary muscle movement, like breathing, is not affected, there is often a sensation of chest pressure, which is why many people wake up from sleep paralysis gasping to take a deep breath. Episodes can last anywhere from 20 seconds to a few minutes.