They do things like "finding that annoying mosquito" and "calling the children for bed", which are clearly nighttime actions.
I've never thought of the "mosquito" action as purely night-time - it seems to happen randomly throughout the day for me. I usually start my game up at around 6.30 AM (before I go off to work) and I think the peeps think it's too early for the kids to be up, so they often go to bed on their own, or get sent there by a grown-up.