I admit that I have almost invariably pulled an all-nighter (or, at maximum, slept for 1-3 hours) before each of my medical school exams. Sometimes, I try to lay down and get some rest, but my mind is going 2 million miles a minute and I can't fall asleep. So, I figure, I might as well study/cram since I won't be sleeping. Although it's horribly nerve-wracking and I feel overtired for that period of time, once I get to the exam, I usually get in the zone, and am able to focus. Only on a couple of occasions did I start to feel seriously fatigued towards the end of the exam.
I don't really think it has impacted my performance in a negative way. I do fine on exams. But I don't know if it's worth the physical and mental anguish that I experience the day/night before the exam. I can't speak for my classmates, though. I know plenty of other people who do it my way, and also many people who need a full night's sleep before the exam.