Why can't I go to sleep?

When I go to bed at night, I am really tired, but then I always just lay there and I cannot ever go to sleep for about three or four hours (no exaggeration, folks), and it really gets on my nerves, because then I will not even be tired, just laying there, so I will get up and go do something, but then I REALLY will not be tired...and that would not irk me so much if I woke up in the morning and felt fine, but I feel like I've been awake for two days, and then I have to got school, and it's just hell. Is that normal..if not, why is it happening?