Exhausted?

Okay I'm 19 and no matter how much or how little sleep I get in always exhausted.i use to be able to stay up all night long. But about a year ago out of no where I was exhausted all the time for no reason. It doesn't matter if I work an 8 hour day or I just lay in the couch and watch tv. I went to the doctors to be tested for hypothyroidism and they tested me for anemia and a few others things as well. All tests came back negative. Anyone have any ideas on what the problem could be or any tips on how to fix the problem?