I would always say sleep unless you have really done almost nothing and need to cram. I mean personally, if I just go over stuff paying decent attention and making sure I understand like a few days before, nearly all of it gets lodged into my head at just the necessary amount of depth, though it might be gone within the week. However, the night before...it'll be a less comfortable, more transitory sort of knowledge, and you won't feel adept with the material in the exam. And going over stuff rapidly and trying to think about everything at once is a sure fire way to make you doubt yourself and hurt the knowledge you do have.