The law of the (lack of) conservation of energy is as follows:
F+S+N+X =/<0, where:
F is the daytime fussiness quotient of the tiny human,
S is amount of sleep everyone is getting,
N is whether the small human is napping,
and X is anything else that can go wrong.
If S is positive, then (at least) one of the other values must be negative, since the total value must be equal or less than zero.
Baby starts sleeping? You will decide to major home renovations and/or your toddler will stop napping.
Toddler starts napping? Baby will start being inexplicably fussy all day, and/or you will get sick.
Getting better? It's tax time, and the older one catches your cold.
This explains why I keep expecting to catch up, and I never do.