My 18mo has recently starting waking up between 4.30-5.30am every morning. She goes to bed at 6.30-7pm and from about 14mo was sleeping through til 6.30-7am.
I've read a bunch of similar threads on here and there's lots of advice on earlier / later bedtimes, checking room temp and black-outs, testing later tea time... but also lots of advice suggests this wake-up might be normal and here to stay.
How long did it last for you? Is there anything you did that helped? Either shift wake-up later, or just help you cope with the earlier starts.