It's not only about being tired enough to fall asleep early. If I stick to a 10pm-6am sleep schedule I feel exhausted during the day, and by early afternoon I'll be falling asleep. It's like being jetlagged permanently; my body simply doesn't want to keep to that schedule. It's not just an "oh, you need to stick to the schedule long enough to adapt and get into a proper routine" situation either - it's something I struggled with for years while I was in school and university, despite getting enough sleep.
It's amazing how much better and more energetic I feel - physically and mentally - now I'm able to keep to a sleep schedule that suits me. Obviously exercising is a good thing, but early/delayed sleep phase syndrome are real things.