That's something really common. The same happens on weekends: On Friday you are at your best, but on Monday you'll be exhausted and tired.
The same thing happens on vacations. For some reason, you'll get more and more tired during a vacation. Especially if you have special reasons like kids. When I'm getting back to work from a vacation with kids, I'm resting at work.
As crazy as it is.
I actually totally understand this (and this is what I internally had in mind with my first message). Kids are like vampire, emptying the energy pool at huge speed. ^^
I thought you might have kids and understand :D
Kids do empty the energy pool fast. But they make parents so happy so easily too:
Only by sleeping, they make parents extremely happy ;)
When kids sleep, parents... sleep too, trying to recover at a much smaller pace :p