I seriously doubt that cramming while tired does any long-term organic damage. But there is loads of evidence that tiredness impairs performance in various cognitive and physical tasks, which obviously will include exams. Try to have an early night, and before that unwind this evening by taking a hot bath, perhaps having a hot drink (something without caffeine) or some milk before bed, and doing something that helps you unwind like meditating or reading a novel. Try to sleep - if it's difficult, it may help if you repeat the mantra 'dreamy, drowsy, heavy and sleepy' to yourself over and over. Even if you're too keyed up to sleep, remember that just closing your eyes and resting in bed for the night is much better than staying up trying to cram when you're too tired to take in new information.