It's most likely that you're waking up a tiny bit and turning it off, then going back to sleep again. It's not uncommon.
There isn't really one single solution for this, but there are several thingsyou could do to help. Try buying a radio alarm or a very loud alarm, and put it on the other side of the room. That way when it goes off, you have to get out of bed to switch it off and that will wake you up a bit more.
If you are at uni and live in halls, why not ask someone else who has a class at that time to knock on your door on the way to the shower or something?
If you do find it difficult to get up after a late night, then it's possible you aren't getting enough sleep at other times: someone who sleeps enough shouldn't find it a problem for just one night. Why not try to get an hour's sleep more each night, and see how that goes?