Hi there!
DS has been a world class night sleeper since early on (sorry!) but he started waking between 4:30am and 5:30am a good few months ago. I go in, make sure he's got blankie and dummy, tell him to lie down, pat his back and leave, all in less than a minute, and he usually naps on and off until he wakes around 6:30am. No obvious signs of anything wrong, I didn't think the room was too light, cold etc
About 2 weeks ago I bought blackout curtains and put those up over his blinds. It's taken about a week, but DS has slept through without the early morning wake up three times in the last five days! And, the mornings he has woken, he's gone back to a deeper sleep rather than waking every 20mins or so. He's even had a couple of mornings where he's slept past 7am! So maybe blackout curtains are worth a try, even if you think that light isn't the issue.
HTH