What I found worked best for me was studying in 40 minute blocks with a 10 minute break to eat or walk around the house aimlessly.
Studying for too long will make you tired and you won't retain it.If you make sure you do a couple of hours each day and do it consistently then you will be fine, but studying from when you get home to when you go to bed like apparently some people do is absolutely ridiculous and ineffective.
Cramming doesn't work. You might get good sac marks but you're going to have to relearn it for the exam. Who wants to relearn the entire course for each of their subjects in the couple of weeks before exams?

Crazy.
Also, if you are really tired all of the time then maybe visit your gp, there could be a medical reason