One of my friends experienced it for the first time this week, and here's the advice I gave him.

If you've ever experienced sleep paralysis, you know how scary it can be. If you haven't, allow me to fill you in on this human phenomenon.

Sleep paralysis is when a person wakes up in the middle of a deep sleep, but they are unable to move and it feels like they are suffocating. Some people hallucinate at the same time, which can make the entire experience absolutely terrifying.

What's really happening is this: during REM sleep, your body relaxes to a point of almost-paralysis, likely to keep us from acting out our dreams. Your breathing is slowed, too.

Sometimes, during this period of sleep, people wake up, but they're not able to speak or move and they're still dreaming...with their eyes open.

Before scientists could figure out what was going on, people used to call it "riding the witch" or the "sleep demon." They felt like they couldn't breathe and said that a demon or a witch was sitting on their chest.

The first time it happened to me was when I was in 7th grade. I was sleeping on the couch in our living room and I opened my eyes. I could see my mom putting jackets away in the closet, but I couldn't move or talk to her and my breathing was shallow. It's happened at least once a year since then, and I've learned how to deal with it.

It can be caused by stress, lack of sleep, PTSD, jet lag and anxiety and unfortunately, there's no way to prevent it. But you can learn your triggers and how to wake yourself up.

For me, stress and lack of sleep seems to trigger it. I've also noticed that it almost always happens when I'm sleeping on my stomach, so I try not to do that anymore.

When I know that it's happening, I try to move my hands. It's hard, because your body is still in a state of paralysis. I start with my fingers and try to make a fist - after that, I force myself to switch sleeping positions. It almost always works...but it's not easy.

Overall, it's super scary and almost impossible to predict. But if you have a plan for when it does happen, you'll be able to end the nightmare quicker.