Waking up early for the day

My 19 month old has been waking up anywhere from 4:30am-5:30am, the usual being 5am, the last 2 months. He has the occasional sleep until 6, like 3-4 times in the last 2 months. He used to sleep until 6:30-6:45am. Today he was up at 4am. Anyone else have these early morning wake ups that eventually resolved? Did you do anything different or did it happen on its own. His nap is 11:45/12pm-1:30/2pm and bedtime is 7pm.