From what I've been told it's more to do with total sleep in a 24 hr period as to what they need. So, what time does she wake from her final nap?? DD at her age now needs to be awake for 3 hours before she will go down for another sleep in her bed - so I wake her at 4 since bedtime is 7. She sleeps for 1.5-2.5 hrs in the morning depending on when she woke up but I always wake her at 12 on the days she sleeps longer for lunch and to ensure I can put her down at 3 so she can sleep for an hour in the arvo - doing this I haven't had any worries getting her to sleep at 7. On one night a week we go out of an evening at 7 and so I let her sleep for as long as she wants on that day for her day naps and don't wake her like I do with the other ones and on that day she'll usually finish her final sleep at 5ish - she won't sleep until 9 at the earliest on those days because she's had so much sleep during the day. Perhaps it's the same with your DD if she's having nice long day sleeps? HTH - GL.