Why are some people Night Owls and others Early Birds? What makes a person either a Night Owl or Early Bird? Is it learned or genetic? Can a Night Owl become an Early Bird with practice? Or is it hopeless??
I believe it is mostly inbred. How ever your biological clock can be changed. I know mine has. Working a swing shift my body never knows what time it is.
Got me man. I’ve been trying to figure this one for a while also. My whole family is a bunch of Early birds, but yet i never was. I work a swing shift and have no problems switching around from waking up at 6pm to waking up at 6am, but i find as the older i get, it’s getting a little harder and harder to switch. But i’m alot more comfortable awake at night then during the day time. Part Vampire?!? 
I tend to go along with the inbred or it’s a part of your biological/genes makeup. Can’t explain it any other way.
My theory is that everyone’s internal clock is a slightly different speed, with some a little slower than 24 hours, and some a little faster. For instance, a person whose internal clock is dead on might, for instance, go to bed at midnight, and wake up at 8:00 AM. Now, suppose my internal day is 25 hours. This means that, assuming the same wake/sleep ratio, I’ll want to spend 16:40 awake and 8:20 asleep per cycle. OK, I get up at 8:00 AM, and my body is telling me it’s time to go to sleep at 12:40. The next day, though, I still have to get up at eight to go to work, so the cycle repeats. It becomes even more pronounced on the weekend, when I don’t have to set my alarm for a couple of days, so by Sunday night, I’m not feeling ready for bed until 2:40 AM. The reverse works for early birds.
And remember, early to rise, early to bed, makes a man groggy, grumpy, and dead 
I think Chronos is on target with the internal day not matching 24 hours. Mine seems to be about 27. I’m good awake for about 17 hours, but I need 10 hours of sleep to be comfortable when I wake up. Needless to say, I don’t get that, so I’m just grumpy for the first few hours every day.
I don’t know how illuminating this is, but I figured I’d add my own experiences to the field reports.
I have always lived in the Eastern Time Zone of the U.S. Since time immemorial (ie–junior high school or even earlier), I have seldom been able to fall asleep before, say 1am or so. Certainly not on a regular basis. My parents thought the moodiness was a sign of my teens – HA! Anyway, I went off to college, and for four blissful years, more or less, I stayed up til 4am or later, and woke up at 11am or later. Never have I functioned better. After a year in the working world, and another year in which the sadistic folks in scheduling gave me 8:30am classes for my first year of law school, it’s back to zombieville.
Fast forward to this summer. I’m working in California. It’s too early to tell the pattern will continue, but oddly enough, it seems that I have simply spent my entire life in the wrong time zone. I now fall asleep without problem around midnight, and wake up around 7am ready to start the work day. Weird, yes? I always thought I had one of those internal clock issues the other posters have brought up. Turns out, I just wanted to be out West.
–Amy
A long while back, a father and son did an experiment where they lived for a month or so in a cave and set the clocks for a 28 hour day. The son adapted fairly quickly to it, whereas the father was constantly irritable and cranky (though it might have just been from living in a dank cave with his idiot son as his sole companion). This suggests that your clock is at least partially learned behavior and the longer you stay at something, the more ingrained it becomes. Interestingly, when a smiliar experiment was done, this time letting people set their own ‘clocks’ in the cave, the average appeared to be a 26 hour internal clock.