How Many Sleeps?

My daughter doesn't quite get the idea that "days" are how we keep track of time. She prefers to track time with how many "sleeps" it has been.

For instance, if I told her we were going to see her friends tomorrow, she would have no problem saying "next year" or "next week" instead of tomorrow. But if I told her that in one sleep, we will be going to see her friends, she would know that we are leaving tomorrow. Go figure.