Has there been any scientific evidence suggesting this is the case?

In college I struggled to wake up in time for 8am classes (that I had no interest in taking). Not surprisingly, a big part of the problem is that I was staying up until 11, or 2, or 4 every night. My typical schedule now (29 years old) is asleep around 9, up at 4:30 and gym 5-6:30 or so. Weekends are only an hour or so later than that, and it's been that way since about 3 years ago.

My point is I think it is as much why you're waking up early as it is a "natural rhythm" or something like that. I would much rather go to the gym at 5:30 than to a liberal arts gen ed class at 8:00. Yes it's possible my circadian clock shifted I suppose, but I think it's as much desire or habit as anything else.