Perhaps it has happened to you.

If not, it’s almost certainly happened to someone you know. A sleeper awakens only to find themselves completely paralyzed. It may feel like a great hand is pressing them deeper into the mattress. The sufferer might even see dark visions—moving shadows, evil things lurking in corners.

Reaching back into the prehistory of myth tells us that this phenomenon has been with humanity for as long as we’ve been sleeping and dreaming. Early belief in demonic possession might be explained by experiences of this medical condition. Nowadays, tales of alien abduction have been associated with such episodes. Even the word “nightmare” has its roots in a 10th-century Persian text describing our subject.

Before we get any further into the darkness, let us assure you: This condition is called sleep paralysis, and though it might feel like you “wake up dead,” we assure you that it’s entirely safe. Here are a few things to remember the next time you wake up completely unable to move.

1. Sleep paralysis occurs when you wake up fully mentally conscious, but without the ability to move.

It usually only lasts a few seconds, although some patients have described episodes that go on for minutes at a time.

2. While we don’t know the exact cause of sleep paralysis, scientists do know how it works.

It all has to do with the body’s natural sleep cycles.

3. Deep REM is the part of the sleep cycle in which the sleeper dreams.

In order to keep the dreamer from acting out her dreams IRL, the body goes into a form of controlled paralysis called atonia. Sleepwalking is often caused by a failure of atonia.

During sleep paralysis, your body remains in a state of atonia while your mind fully awakens.

4. Many people describe a sense of pressure on the chest during episodes of sleep paralysis.

This is nothing to worry about. Involuntary functions, like breathing, are not disturbed by this condition. Still, the sense of pressure may lead to sufferers gasping for air when they finally emerge from the paralysis.

5. There are two types of sleep paralysis: hypnagogic and hypnopompic.

When paralysis occurs while you’re falling asleep, it’s called “hypnagogic.” If you get hit with sleep paralysis while you’re waking up, scientists use the term “hypnopompic” to describe the experience.

6. Just to make this even scarier, lots of patients report a sense that something frightening and unfriendly is in the room with them while they mentally struggle to get their bodies working again.

A small percentage of people who experience sleep paralysis actually undergo full-blown hallucinations. Be grateful you aren’t one of them (assuming you aren’t, and if you are, we’re sorry).

7. There seems to be a link between sleep deprivation and episodes of sleep paralysis.

The cure might be as simple as going to bed earlier. If you have chronic sleep paralysis, it’s probably time to discuss it with your doctor. It might be a symptom of an underlying sleep disorder.

8. Just remember: Sleep paralysis cannot hurt you, and it will pass.

You might want to memorize that sentence so you can chant it to yourself next time you suffer from a spell of sleep paralysis yourself.