I used to sleep all day if I had no motivation, no need to get out of bed, and when I felt down.
I agree exercise would help, even if its 'just' a 10 minutes walk.
Have you got things to get out of bed for, things to do? Apart from studying I mean, which is of course important, but you also need you time and to do things that will make you feel good and positive and therefore maybe help you feel relaxed.
A regular sleep pattern will help, going to bed at the same time each night and getting up at the same time in the morning, dont sleep in and try to avoid napping in the afternoon, that was one of my things and then I'd wonder why I was still up at 2am
Hope this helps a bit and good luck