Get as much sleep as possible the night before the exam. You need to be rejuvenated and your mind to be renewed, and that won't happen if you are tired. While you're cramming for the exam, your mind is forced to study and take in so much that it literally becomes exhausted. If you don't rest your mind, then there is no energy left for it to access all the data you crammed in it. It's there but it's too tired to think. So, you need to rest your mind so that it can have the energy to access everything you put in it.