I think at least part of this is age and habit. In college my best period of concentration and work was roughly 11-3 or so. I was also ~20 years old and had trouble going to bed before midnight or waking up before 9.
A decade later and after military service the lion's share of my day job work is done between 7 and 10 in the morning, and it's rare that I'm not up, dressed and showered by 6:30am or asleep by 10pm.
Individual experiences vary widely; I am 41 now and still have just as much trouble going to bed before midnight or waking up before 9 as I did in my 20s. It still takes effort to restrain my impulse to stay up all night, in fact, whenever there's something remotely interesting or creative I could be working on instead of sleeping. It is not simply a matter of habits, either; I spent most of seven years getting up in time for a 7 AM conference call every work day, and it never got any easier. As soon as that was over, I went right back to my old schedule.
Perhaps it's less of a struggle to fit my life into normal business hours now, after years of experience trying to do it, but that's really just a matter of coping skills; my body clock remains as stubbornly nocturnal as it has ever been.
A decade later and after military service the lion's share of my day job work is done between 7 and 10 in the morning, and it's rare that I'm not up, dressed and showered by 6:30am or asleep by 10pm.