Hypnic Jerk

Do you ever suddenly jolt awake when you're going to sleep?
I do and at the time it gives me a fright but I don't usually think anything more of it and go to sleep. Steve sometimes notices me twitching in my sleep too.
Last night when I was just trying to go to sleep, I kept jolting every few minutes so took a sleeping pill because it was annoying me. Today I thought to myself, why do I actually do this??

Apparently it is called Hypnic Myclonia or Hypnic Jerks. They happen when we begin to drift off to sleep as the muscles are relaxing. The brain thinks that we are falling or dying and trys to signal the limbs to wake up. This can happen in the REM stage of the sleep cycle (the eyes often twitch too during this time but we don't notice that.)
Usually only 1 jerk will happen a night but some people have many and that can be called periodic limb movement (PML). It is often caused by over tiredness or stress and possibly medication.
People who jerk awake often feel as if they are falling or have had a falling dream. I have never had a falling dream and dont think I have felt the falling sensation either.

Here are some accounts of what other people have experienced:
http://www.wisegeek.com/what-is-a-hypnic-jerk.htm