This has probably happened to you. You're asleep, and you're dreaming. Maybe it's a glorious dream about a regal unicorn with Donald Trump's hair. Or maybe it's a scary dream, and you're being chased.

You start to wake up, and you can see the clock on the other side of the bedroom. You're supposed to be awake, but the dream continues: You can see shadows in the room, and you're still being chased.

Then you have the terrifying realization that you can't move. This is called sleep paralysis, and it can be truly frightening.

ATTN: talked to Terese Hammond from the University of Southern California's Keck School of Medicine about sleep paralysis.

"It's a fairly common and in most circumstances harmless, but it's a really scary event," said Hammond, an assistant professor of clinical medicine in the division of pulmonary, critical care and sleep medicine. Up to 45 percent of people will have an incident of sleep paralysis at one point in their life, she added.

So what is actually happening during sleep paralysis?

There are two types of sleep: Rapid Eye Movement, or REM, sleep and non-REM sleep. About 25 percent of your slumber is REM sleep: that is when about 80 percent of dreaming happens. When you're in REM sleep, your body goes into a state of paralysis called REM atonia. That keeps you from physically acting out your dreams while you're asleep.

"Sleep paralysis occurs when the line between awake and sleep is blurred," Hammond said. This usually happens when people are falling asleep or waking up. The atonia of REM sleep lingers and carries over into non-REM sleep. If the person wakes up, and the paralysis is there, things get freaky. People are dreaming vividly, and they can't move, but they're still aware of their surroundings in reality.

"Most people who have this happen will know exactly what they were dreaming," Hammond added. "They remember every detail of it."

Such paralysis happens rarely for most people, usually triggered by a lack of sleep beforehand, or alcohol and drug use.

"Many people experience sleep paralysis for the first time during their teenage years, when they're really sleep-deprived and experimenting," Hammond said.

Consistent sleep paralysis can be a sign of a more serious disorder called narcolepsy.

Narcolepsy is a neurological disorder caused by the brain's inability to regulate sleep-wake cycles normally and is often associated with sudden sleep attacks, according to the National Sleep Foundation.

But not all narcoleptics fall asleep standing up, as the stereotype suggests. In fact, most just feel incredibly tired throughout the day. Consistent experiences of sleep paralysis could be a symptom of narcolepsy. If you have them, you should talk to a health professional.

There's also the possibility that sleep paralysis can affect your sleep patterns. "People can actually have something like a post-traumatic stress reaction and be afraid to go to sleep," Hammond said. "They can remember the context of those feelings. What's important to remember is that sleep is very fragile."

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