Could have been me starting this thread!
15 mo DD is exactly the same...from 6 months to around 13 months she woke at 5am..couldn't get anything later...then out of the blue @ 12/13 months she started sleeping later...until 7/7.30 and sometimes 8am - one theory was that as she had started walking she was just a bit more wrecked - and she walks around all day long - is a really active girl! She also dropped the second nap at this stage (and would usually only sleep for 1 hour, maybe 1.5 on a good day for that one nap). But because she was sleeping later I got her into a 12 hour day so it was a good 7-7 schedule.
But....the last two weeks she has reverted to 6am starts - again, no idea at all why the change. She got some new teeth during the 'good sleep' stage so it's not that (I think!) - and her eating habits have declined the last few weeks aswell. She has always been a great eater - great appetite and would eat all types of fruit, veg and meat but not only is she now eating less but is much fussier about food - refusing things she used to love.
So I'm wrecked again every day and dealing with her flinging alot of the food I cook on the floor - frustrating due to the stress about the amount of food she is consuming, the cost of the waste and the time spent planning and preparing her meals!
After that big rant (sorry - someone doing loud housework outside their house woke her early from her nap this morning so she is wrecked and I know she won't go to sleep now until bedtime) I can't offer any help except sympathy!