It really depends on which year you're referencing. During the first two years of med school, with the exception of exam periods, there's no reason why you shouldn't get a decent amount of sleep.
In third year, however, it varies tremendously with the rotation. During clinic months, about 6-7 hours a night. But there were times on Medicine and on OB/Gyn, specifically call nights, when I just never went to bed (literally). And on off-call nights, there was still so much work to do that I was lucky if I got four hours nightly.