Our toddler (23.5 months old now) has been waking up between 4:15 and 5:30 every day for 3 months. (Occasionally he'll wake even earlier -- 1:00-3:30 -- but we can usually get him back to sleep then. If it's after 4:30 he never goes back to sleep.)
When he wakes, he gets out of bed and starts crying immediately. But if we go to him, if it's after 5:00, he's usually pretty happy and ready to play, though earlier than that and he'll sometimes be very cranky and fussy.
Before this started, he slept pretty reliably from 7 pm to 6-7 am for about 6 months, which was great. He has been nicknamed "baby no-naps" since birth, though -- he is very difficult to get down for a nap, and will never nap for more than an hour without waking (though if his mom nurses him when he wakes he'll fall back asleep for up to another hour.) Never in his life has he gone down for a nap on his own, it's always been holding him or nursing him or in a car. We've tried sleep training for naps but he just cries the entire nap time and ends up missing then nap altogether. He's got a big bed now because at around 22 months he became capable of climbing out of his crib even at the lowest level, so he can't be contained.
Bed time, though, is great! We read him a few books, turn off the lights, put him down in bed and say good night, and he goes right out without complaint. Unfortunately, he then sleeps only about 9-10 hours.
In general, he always seems to be trying to shift his schedule even earlier. He'd like to take his nap by 10am, though on day care days he has to stay up until 12:30 (at which point he instantly falls asleep in the car.) On days when he wakes up fussy he's been known to nap as early as 7am (which results in a 2-nap day with a second nap around 3pm.) Normally we let him go down for a nap sometime between 11am-12:30, at which point he'll nap either 1 or 2 hours. It's then a struggle to keep him up until 7pm for bedtime -- he starts getting tired around 4:00 and would probably go to bed by 5:30-6:00 if allowed (but if we do allow it he's up by 3-4 am and ready to go.)
If we put him to bed later, he still wakes up 4:15-5:30. When we travel to visit relatives (3 time zones east), he'll happily go to bed at 10pm local time (so 3 hours later, really the same time as at home), but wakes up around 6:15-7:30 (so only 2 hours later.) He'll shift that wakeup time about an hour a day, so by 3 days of visiting family he's on the same 7pm-5am schedule as at home.
It's been 3 months now. We've tried longer naps (2 hours/day), shorter naps (single 1 hour nap a day), one of those clocks that lights up when it's time to get up (doesn't do much good because he's never slept long enough to see it), just putting him back to bed & leaving the room in 15-minute increments (he just cries continuously standing at the door until we decide it's time to get him up), staying with him or nursing (he's happy but not the least bit interested in going back to sleep, he's never once closed his eyes again after one of these wakings), later bedtimes (he still wakes at the same time), even cosleeping (he wakes happy, but still at the same time ready to go.)
I'm not sure what else to try; he seems to just want the day to be shorter, with an earlier bedtime, earlier naps, and earlier wakeup every day. I worry it might just be genetics -- my wife is a morning person who only sleeps 6-7 hours a night, and I'm a terrible sleeper (I had insomnia and night wakings until 9 years old, and am frankly not a great sleeper even today.)