Toddler Mystery #1

How does a child go from unconscious in the car to completely awake in the course of a diaper change? I've been trying to get Annika to fall back to sleep for going on an hour. I think she's winding down, but I still hear chattering from upstairs. Do I pick her up, or do I keep her in bed? Is she somehow just not tired? Impossible. She will fall asleep . . . .