Also, "nights" are not a real thing, not in the biological sense. In the surface ship nuclear power plant world where there is no air conditioning, you can't stand watch for more than 5 hours, which means 4 5s and one 4 hour watch. Think you stand the same watch every day? Nope, not enough people. You normally stand 1 on 2 off (5 and dimes) or one on 3 off (5 and 15s). This puts your sleep window at different spots every night.

Subs are air conditioned, so they can stand either 6 and 6s or less commonly 6 and 12s. Their lives are generally worse in every way. Except they all like smelling each other farts and sharing beds, so it works out.