What causes me to sleep so much?!

It's probably a really dumb question but I'm going to ask it anyway. What causes someone to sleep alot? If I don't sleep all the time, I'm completely beat and I shouldn't be. I thought maybe I was dehydrated but I drink enough to not be, so that's not the case. I also tried cutting back on my sleep by only getting five hours and I was still tired. I'm not pregnant, I don't work long shifts or do anything to be this terribly tired. Even if I sleep ten , I'm give out and have to literally fight myself to keep my eyes open. My grades in school are decreasing dramatically because I sleep all the time. I'm considering consulting my doctor if this continues but I wanted to check here first and see if anyone had any ideas.