Take a weekend off - and I'm not kidding, just take a trip down to along the Great Ocean Road or to the Dandenong Ranges and just have a good weekend out - you'll see that you feel much better when you come back. This happens when you try to study for too much and too long - I usually find that whenever I can't absorb information anymore, I just go out for a drive, crank up the music and just drive around for 15 mins and come back home - it does wonders.
I know it's probably better for my health to go for a walk or a run, but I don't feel safe at 10.00PM and such going for walks and runs, so drives are better - that could also be an option for you if you have your license.
I think it's just about finding something you enjoy and taking a bit of time away to do that - maybe you play sport - take a weekend off to focus on your sport and training - for example.