Not to start another "shift hours" debate, but I find that it really depends on shift length, schedule, etc.
When I was an MS-3/4, I did 12 hour shifts (7am-7pm) and there was some serious fatigue going on, especially after say 4-5 night shifts in a row. I spent all my time outside the hospital sleeping. Now as a resident, we do 10-hour shifts and life is way better. It also helps that we use a 'western progression' schedule. For example; I'll work two morning shifts, two afternoon shifts, have a day off, then have two night shifts, and have a day off after that. Repeat. Its awesome because you're always "working later" and "getting to sleep in". Correct me - all those out there in attending-land, but schedule is going to be a big consideration when I take my first gig out of residency.