My son has always been an early bird, but he went from getting up @ 5:45 or 6:00 to getting up at 4:45-5:00 when he was @ 20 months. Since we also had a newborn, the whole family was exhausted and it was miserable!
The only thing that seemed to help was to shut his door. I think if he woke up and the door was open, he would think... "I can walk out there and do things!" If it was closed, he must have just rolled back over and went back to sleep. That seemed to help a little but was by no means a miracle cure. He is now 30 months and getting up @ 5:30-6:00.
We tried putting him to bed earlier and also later, and neither seemed to work consistently. Putting him to bed later backfired alot, because if he still got up early, he was MISERABLE because he was extra tired.
Good luck! You are not alone! :)