I had this problem, too, with my daughter around the same age. Actually, it's starting up again. Anyway, here's a link to the responses I got from other moms.
http://www.mamasource.com/request/18232268937549053953/69...
I think it was the sun waking her up because as soon as we switched to daylight savings, she started sleeping later. But now that the sun is coming up earlier, she's back to rising early to (about 6 am). She goes to bed around 8 pm every night and rarely misses her nap, so I think it's just something we have no control over. She does occasionally wake up around 5 - 5:30 am but I just tell her she has to stay in her crib and she does until about 6 am when she starts crying to get out again.
If you can, try curtains or blackout shades in her room if you find that it's the morning light waking her up.
Good luck, I know how exhausting it is to be up so early.