A child's natural wake up time is only apparent when they have consistently started sleeping a long trajectory of 9-10 hours.
Here is a list of possible reasons for early morning awakenings and sporadic awakenings could occur singly or in combination:
1) Over Tiredness - if otherwise technically still sleeping long trajectories but woke up before 5:30 it could mean bedtime was too late.
2) Nap transitions - Early morning wake ups during all nap transitions occurs in 95+% of otherwise well-rested children that sleep through the night
3) New milestones - at the peak of new milestones or just before mastering something new (rolling/ crawling/ walking/ start of terrible 2's), Early Wake up would be the norm for 2-3 weeks. The spatial goal of a skill works over time during sleep.
4) Major teething - e.g. top front teeth and molars. The week before the teeth cuts through when the gums are jelly and rough to the touch is at it's worst.
5) Seasonal - even if none of the above is applicable to the child in question, the natural wake up time of ALL well-rested independent sleeping kids do shift earlier with the earlier sunrise. The effects of the changing season would be noticible by May and at it's worse in June (around summer solstice) for those living in the northern hemisphere. Things would noticibly be moving "later" again by end of July.
All the above do cause early risings on there own or in combination. The above typically describes children who are otherwise well-rested, consistently sleeping through the night and practice independent sleep.
6. Genetic - No matter how attuned the routine there are those that will wake between 5:00 and 5:30 a.m. It starts to present itself at 5-6 months. These babies and children tend to take longer naps, transition to fewer naps two months later than babies of the same age, and once they arrive at one nap a day that nap can be up to 3.5 hours. A later rising will only happen when the child has reach full sleep maturation which can take 3-4 years. There are wonderful qualities of the early riser in both temperament and personality which makes it even harder for the exausted parents that are not naturally early risers. A study in the August issue of the journal SLEEP reports the identification of a gene mutation that may allow the carrier to function normally on less than six hours of sleep per night. The genetic variant also appears to provide greater resistance to the effects of sleep deprivation.