When you are exhausted and go to bed your body shuts down faster than usual. That 'jerk' or 'kick' that your leg makes is a signal from the brain to make sure you are not dying. Your body takes a a while to slow down to sleep but whilst exhausted your brain double checks to make sure you are still alive by sending a message from your brain to to the bottom of your body to check if your nerve sytem is still functional. This is usually a quick kick of your predominant leg.