Where I am, the sun has started setting REALLY early, and we still have months to go until the shortest day of the year... And then if it's a rainy day? Oh boy, watch out covers, here I come. But the weather and the lack of sunlight could be having an affect (effect?) on you. You could investigate getting one of those fake sunlight lamps. I sleep with my blinds open so any possibility of sun can come through in the morning (but then, I also have a tendency to sleep with a pillow on my head... So those two might be canceling each other out).

And this is going to sound trite, but I've found exercise has helped with my energy levels. It seems a little counter-intuitive... expend energy to get more energy... But apparently it works. I know in the winter that gets a lot harder.

Do you like what you're studying? Do you find it interesting? It's a lot more difficult to get excited about studying something you have no interest in.

I definitely had to stop doing homework sitting in my bed (in an apartment that's a lot easier than in a 6' x 6' dorm room). I've taken so many naps with so many of my textbooks...

Just keep trying. I'm sure your body is going to resist any changes right now, but I bet if you can keep at it, you'll pull through. And even after making a bunch of changes and you're still having trouble, I'd go back to your doctor, either a pdoc or maybe a GP.