As father of three (a 3 year old and 1 year old twins), I've found that when we only had one kid, our schedule remained basically the same. But now with three, we are somewhat forced to shift toward being morning people. With multiple, good luck getting them back down to sleep if the sun is up. By the time you succeed, the others will decide to be awake. Also morning routines take longer, so you need to start your day earlier to compensate.

I feel like a big part of it is that very young children are geared towards being early (at least from my perspective) risers. Parents end up shifting their day to account for that. And then as kids get older, their rhythms start moving to starting the day later, but by that time the parents' routine is molded around being up early.

Not sure where I'm going with this exactly, just an observation of mine.