This is definitely My personal favorite  Every single time I am bone cracking tired, my kids lose all the interest in falling asleep. They want to run around, with me following them, persuading, coercing and even threatening then to go to bed, but they run away laughing & giggling.

and sometimes, when I am all fresh they surprise me by sleeping so early!

so I think, in the laws of parenthood:

Time required by a baby to fall asleep ∝

Tiredness of a mother