One of the most common mistakes that I saw my friends making in college is slacking on their sleep. All too often I would see my friends stressed out about whatever was going on in school and think they should drink caffeine and stay up all night studying instead of sleeping. However, once your brain is at that point of exhaustion, you are not really absorbing anything at all anymore! Once you're at this point, go to bed! I promise that getting a good night's sleep will help you accomplish so much more during the day than you can if you are just run down all the time. I mean, I doubt you will be able to maintain that perfect 8 hours per night, but if you can get at least 6 hours of sleep every night with a little more on weekends, then I would say you're doing fine.
It's all about time management in medical school. You should not depend on "cramming" before a test. Study hard and consistently, and you will have time for sleep. I agree with Dan. Sometimes you won't get as much sleep as you want. I think residency is much more demanding of your time, and that's where you are more likely to be sleep deprived for a while.
I am currently in medical school and I make sure to set time in my schedule for at least 7 hours of sleep at night. I rarely crammed unless it was a week of finals or midterms. I usually checked my calendar in advance and set time to study for exams that may be a week ahead or so. Maybe in the beginning when you're getting used to the curriculum and course load you may lose some sleep. Studying is an adjustment and figuring out the best way to study for you may take a few trial and error efforts where you may lose some sleep.
The first two years are similar to college. You have to master time management. Study consistently throughout the course. You shouldn't be pulling any all-nighters. Getting the right amount of sleep will improve your grades more than cramming.
During the third and forth year there are rotations with overnight shifts. Hopefully you will get a chance the next night to recover from the lost sleep.