Teen Wolf, Season 3

Stiles:
Have you ever had a dream where you feel like you're about to wake up, but you can't move or talk? It happens because during REM sleep, your body is basically paralyzed. It's called muscle atonia. That way, if you start dreaming about running, you don't actually start running in your bed.

Scott:
That makes sense.

Stiles:
But sometimes, your mind can wake up before your body does. So, for this split second, you're actually aware that your body is paralyzed.

Scott:
And that's the terrifying part?

Stiles:
It turns your dream into a nightmare. You can feel like you're falling, like you're being strangled, or, in my case, like you're in the center of a grove of magical trees where human sacrifices took place.