Personally, I overload during exam period, often going to bed at 2-3 in the morning sometimes 4 and waking up by 8 to be in lessons for help and repeat. it's a vicious cycle but thankfully only temporary but a lot of my friends always comment on how they would never be able to survive on such little sleep.
The issue I had was last year I knew I was ****ed, so it was either do the work or let decent grades slip through my fingers. Not everyone is so ill prepared, of course.
But how much sleep do you get before exam day or even during exam season?