y do I jerk when I sleep

You’re just about to drift off into a deep sleep when suddenly you feel like you’re falling or rolling downhill on a roller coaster. Then your body jerks suddenly and you’re awake. Weird! What’s going on?

This body movement, also known as a sleep start, is what doctors and scientists call a hypnic (say: hip-nik) or myoclonic (say: my-uh-klah-nik) jerk.

These jerks usually occur before you enter the deeper stages of sleep and are completely normal. Doctors and scientists aren’t really sure why they happen, but they have a few theories.

One theory is that your brain misunderstands the sensation of your muscles relaxing as you drift off to sleep. It’s normal for the muscles to relax, but the brain gets confused, and, for a minute, thinks you’re falling. In response, the brain causes your muscles to tense up so you “catch yourself” before you fall down.

These jerks can wake a person up, or the person might just keep on sleeping. They’re nothing to worry about, so if one of them wakes you up, snuggle back under your quilt, and try to catch some more ZZZs!

http://kidshealth.org/kid/talk/qa/jerk.html