Im ALWAYS tired?

Im 17, I sleep a good 7-8 hours a night, my eating habits are fine; but i feel very tired all the time. I wake up and feel totally drained of energy and it pretty much continues for the day. I used to drink coffee which helped but i stopped a long time ago. I play tennis a few times a week, but i feel that feeling like this is really limiting and i would be alot more active if i had more energy. Anyone have an idea to as why I am always so tired? Any suggestions? (ive gotten bloodwork done and according to the doctor its all completely normal..) -- I have been tested for mono