Staying up the night before to cram is the WORST idea for exams. Really. I can't stress this enough.
Getting a good night's sleep for the exam is vital.
I'll put it this way: I once missed one of my University exams entirely because I stayed up all night rechecking things, thought the exam was in the morning... but the exam was actually in the afternoon, and by the time it rolled around I'd already passed out from exhaustion. I had to re-do a year of Uni because of it.