Etiquette

People work different shifts here, both because they're assigned to one of the 3 regular shifts or because they need to put in long hours to maximize the amount of work they get done down here. Because I'm supposed to be on "night" shift (hard to call it that when the sun's out 24 hours), I woke up at 11:30PM, got ready and headed into the station for breakfast. I met a couple of other IceCube people who were just leaving to go to sleep. What's the proper greeting at that point? Good morning? Good night? A lot of people just say "Hey", unless they know what stage of the day that other person's at.