Often, as I'm going to sleep I suddenly jerk awake. And sometimes it feels like I'm falling down. What's going on?

This body movement is what doctors and scientists call a hypnic (or hypnagogic) or myoclonic jerk. It's also known as a "sleep start," and it can literally startle you out of falling asleep. This type of feeling is normal, and it can happen before people enter the deeper stages of sleep.

Doctors and scientists aren't really sure why our bodies do this, but they have a few theories. One is that the brain misunderstands what's going on as our muscles relax before sleep. It's normal for the muscles to relax, of course, but the brain gets confused. For a minute, it thinks you're falling. In response, the brain causes your muscles to tense as a way to "catch yourself" before falling down — and that makes your body jerk.

These body jerks can wake you up with a start — but they're nothing to worry about. Lots of people fall right back asleep afterward.