Yeah, I found myself wondering that a lot. But I guess we're only watching when (1)the main cast is awake, and (2)when something interesting is going on. They must sleep sometimes.
I think it's just that, compared to other series, you don't see their quarters as much, and you don't see them resting much, so it kind of seems like they live on the bridge (or are on other planets, etc) all day and night.
We know from Kirk's driving in "A Piece of the Action" that the Enterprise did not have a shift.
Heh. This. I enjoyed that part.