I'd say that the true measure of a person's preferred sleep schedule can be assessed by looking at their preferred hours of repose while they are in college. (Be warned, I am speaking in generalities here...please accept that before reading further.)
No early-morning high school classes...most of the time you can set your own schedule, at least to a point. No 9-5 job requiring report to duty early in the AM or at any specified hour. No babies or small children to keep you up all night or to wake you up early. At least for me, my years of college and grad school were more free and flexible than any I had before or have had since by a long shot.
As a college student, I went to bed around 2AM and woke up about 7AM most of the time. That was all the sleep I needed (yes, it amazes me to this day, as I stumble about groggily and demand coffee when I get anything less than seven hours' sleep.) Back then, the late hours of the evening and early morning were my preferred stomping ground: I would often study until midnight and then go out!
Then, because the universe has a sense of humor, I married another night owl and had children. (Cue the laughing.)
My sons are the earliest of early birds. My daughter is a night owl like her parents, but the boys are so loud that it is very difficult to sleep much later than they do. The rest of us have started going to bed early as well purely in self-defense. If I ever catch the proverbial early bird, I'm going to whomp it with a stick!
Today's NaBloPoMo prompt: Are you a morning person or a night owl?