After a long day of studying countless hours preparing for exams, it is important that you take your time off, and let your brain get a little bit of fresh air. Remember, the brain is like any other part of your body – if you overwork it, it will end up hurting and there will be a performance drop.
Take a short nap, get some snack, drink some juice, strech or exercise, go for a walk, have some laughter.
Let yourself relax for, say, 15 to 40 minutes. Of course, your resting time should not be as much time as your study time. But jump back into studying after enough rest, your brain will be prepared to take the next 3-4 hours batch of studies.