For some people, sleep can actually attack.
As many as 200,000 people in the U.S. are thought to have are thought to have some form of narcolepsy, a chronic sleep disorder in which the brain's ability to regulate sleep-wake cycles doesn't work correctly.
"I might look asleep when I collapse, but I'm fully conscious," says Julie Flygare, a 29-year-old with the condition, in the CBS LA video above. "It feels like being awake inside a corpse."
People with narcolepsy with cataplexy have sudden bouts of sleep paralysis that are triggered by feeling strong emotions, like laughing during a movie or at a friend's joke, Emmanuel Mignot, a narcolepsy researcher at Stanford Center for Sleep Sciences and Medicine, told The Huffington Post.
"REM sleep gets mixed up with wakefulness," Mignot said. "You can't move because you're paralyzed from REM sleep, but you're actually awake."
Flygare was diagnosed with the sleep disorder eight years ago. Since then, she takes medication and daily naps to help cope. She's also started a blog on the topic. She's written a book. And she's started a nonprofit, Project Sleep, to raise narcolepsy awareness.
"Narcolepsy is everywhere," Flygare says. "No one out there should feel isolated."
Hear more about how she deals with the terrifying condition and what she says is one of the toughest aspects of living with the disease in the video.
Sarah DiGiulio is The Huffington Post’s sleep reporter. You can contact her at firstname.lastname@example.org.