It happens mostly when you are REALLY tired, I can't explain why, but it is called Myoclonic Jerk or Hypnic Jerk. It is when you are really tired and your body tries to keep itself awake. That made no sense, well it's like, your brain is falling asleep, but your body isn't asleep yet. That's pretty much how I understand it anyway, it was on House and I asked my dad who is a doctor if thats what it really is and he said yeh.