I think it probably depends on the alert condition, as Defence Stations would probably still be a step back from action stations. Assuming then that the bridge would require at least two or three personnel at each station when fully manned (eg. Tactical Officer and an Assistant TO), you could possibly split it up so that instead of standing six hours in eighteen, people were standing six hours in twelve whilst on defence watches, then pair of the ATO with whoever normally stands the third watch, and the TO with, say, a gunner's mate or other experienced rating (this actually becomes more difficult to balance without at least a few midshipmen to scatter around the place). Six hours on and six hours off would obviously be hard on the crew (or, take the RN approach and extend it to twelve hours on and twelve off - and bear in mind navy vessels would keep that up for months on end when in an active war zone), but it at least allows some rest whilst keeping the ship in a heightened state of alert.

I think that's probably the way to operate it in terms of working around the watches. Question is: how do you organise the midnight meal? Presumably breakfast, lunch, and dinner would all be catered for per normal, which is three watch transitions, but what about number four. Do you a) run it like lunch, which would mean essentially needing to run a watch system as well? Or do you b) basically make it a cold meal of sandwiches etc. prepared in advance, which would mean being able to reduce crew size (and thus bunking requirements, consumables requirements, etc) by only having a single shift (maximum two) in the galley?

When thinking on that it is probably worth considering three things: first is thatthe watch system is set up so that no-one is going to be coming off-watch to sandwiches day in and day out. The second is that the kitchen crew, even if it was on watches, wouldn't be able to follow exactly to the same rotation as the rest of the ship anyway as the last thing you want to be doing is trying to switch over personnel in the middle of service. The third is that a lot of kitchen work, particularly during service, is basically run on muscle memory (which is why back of house seriously considers stabbing anyone who wants modifications to their order as it throws that off. Seriously, just pick the tomatoes off), so its one of the few places it is probably better to let people work the same slots day in day out.

As you can probably guess my leaning would be toward running a reduced number of pannels in the galley. Say, two eight hour watches, handing over after lunch service; which gives the watch doing breakfast an hour to get in and cook, and those doing dinner service two hours after to clean up and do prep for breakfast the next "day".