This worked wonders with our early-rising son, though your mileage may vary. When our son was two or three, he had the maddening habit of awakening between 5 and 5:30 in the morning. We knew he was still tired but nothing worked to get to go back to sleep.
Then I decided to try setting up a clock radio in his room, set to a classical music station on low volume. We told him that it was ok to wake up when he heard the music. At first we set it to his normal wake-up time. After a few days we moved it a few minutes later, then a few minutes later a few days after that. Each night at bedtime we reinforced the message "in the morning, wait for the music, then you may get up." The volume was low enough that he could hear it only if already awake.
After a couple of weeks he reliably slept until 6:30 or 7–a huge improvement.