Psychologist Jade Wu, PhD, writes for Psychology Today about the biological mechanisms behind sleep paralysis.

Sleep paralysis is a fairly common experience—almost 8% of the general population has experienced it at least once. But if you’re a student or someone with a psychiatric diagnosis, your chances of experiencing it go up to almost 1 in 3.

Rest assured, sleep paralysis is usually harmless, especially if it only happens rarely. But why does it happen at all, when is it cause for concern, and how can you prevent it?

While your brain waves may be very active, your body is immobilized during REM. Other than the eyes moving around a lot (hence “rapid eye movement”), your muscles lose tone. This is your body’s way of preventing you from acting out your dreams.

So, every night during REM, you are “paralyzed.” Sometimes, though, the veil between sleep and wakefulness becomes thin and you find yourself straddling both at the same time. 

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