Have you heard of the book "The 90-minute baby sleep program"? It talks about their sleep cycles and how they go through an entire cycle in 90 minutes. Therefore, they sleep, and stay awake, in 90 min increments. So they either sleep 90 min, 3 hrs, 4.5 hrs, etc. She claims this also applies to their wakeful periods. The book has logs and sleep plans as well. I never followed through with completing the logs, but ever since I read it I have noticed the 90 min segments in my DS's sleep and wakefulness. There are, of course, times where he wakes or sleeps not on that "90 min time line" but it might be something to look into in order to understand and extend your DD's sleep. Good luck!