Crew sleeping?

[Torpedo Death]

I've noticed that the 3 shifts of my crew doesn't always sleep at the same time. What I mean is that sometimes an entire line will be asleep, except for maybe the deck watch crew. Then later, the deck watch crew will sleep while the rest of that shift is on duty. I was just wondering what governs this? I don't think I've moved crew members from one shift to another. But even if I did, I guess I would expect them to just take up the sleeping time of the rest of that shift?