I get that: nerves, excitement, apprehension. I'm dying to get into the exam and look at the paper and apply my knowledge. It's not a matter of how much revision you've done.
To get round this, I just lie there. I think to myself "I'm resting, and resting is pretty much as good as sleeping. If I just rest, let my mind tick and my body relax it'll be fine. How much sleep do I need?" and with that I generally drop off to sleep in no time. As long as you're saying to yourself that even lying there is alright, you should be fine. Remember, you don't need that much sleep before an exam. Ideally, yes you should sleep enough but it's not the end of the world if you don't. Thinking it will affect your performance and it's more likely it will!