I used to think my former roommate and I lived in different time zones. Left to my own devices, I'll go to bed at around 11:30, and wake up between 8:30 and 9:30. Roommate, however, would go to bed at around 3:00, and wake up around 12:30 or 1:00. This meant that on weekends we had a lot of time where one of us was awake and the other was asleep. Roommate was, unfortunately, the type who didn't much care if she disturbed other people, but if she was disturbed, it was a federal case. She'd get annoyed at me for making noise in the kitchen at 11:00 on a weekend morning (not lots of noise, mind you, just normal cooking noise), but would regularly come in with friends at 2:30 in the morning, talk loudly, slam doors and play music, and then seem confused when I stumbled out of my room to ask them to keep it down.
I'm so glad I live alone now. When BF is over, he'll sleep until 12:00 sometimes, but me being up at least 3 hours earlier doesn't bother him. The cat, of course, enjoys jumping on my head in the morning, but if I ignore her she gives up and goes to sleep again.