Well, there's no reason to waste daylight at the least usable time. While it doesn't mean much in a 24hour society, the vast majority here are sun worshipers.
Daylight sets many biological clocks. I believe that there's research to suggest that the lack of it is an element in "winter depression" that some are subject to on a recurring basis.
It would make sense to sleep through as much darkness as possible. We are not nocturnal creaters.