I've got this too, been getting an average of 6 hours sleep each night for the past week - which might not actually sound that bad to some. I get woken up by the most quiet noises, my heart then starts racing, my chest gets sort of pained, then I lie there for a while thinking about having to get up in a couple of hours anyway. It's a stress thing that started with my AS's - I had one day last summer with 5.5 hours of exams in it and I thought I was going to have a heart attack the night before when I managed about 2 hours of sleep after contending with a racing heart/breathing problems throughout the rest of the night.
I've been trying to cut back on my revision - for stress reasons - and keep a more regular sleeping schedule. All to no avail at the moment.
I think it's this thought that if I muck up this summer it will effectively be the end of the hopes I had for myself going to a great university. If I could accept the fact that this really is not the case, I'd do a lot better.