It looks like the 18-hour watch is slowly becoming a thing of the past in the US Navy submarine fleet - the crew of the Los Angeles-class USS Providence recently returned from a 6-month deployment where a new 24-hour watch schedule got great feedback from the crew and commanders.
The 24-hour watch cycle, which was first allowed for submarines in 2012, has been shown to better match the circadian rhythms of the sailors, according to research done at the Naval Submarine Medical Research Laboratory in Groton, CT. The 24-hour schedule means that sailors can sleep at the same time each "day" - with the 18-hour schedule, a sailor may be required to sleep at 6pm one day and then midnight the next. This sleep schedule is similar to flying six time zones to the east every day, and resulted in a sort of permanent jet-lag for crew members.
Under the 24-hour watch schedule, sailors are on watch for 8 hours at a time, instead of the 6-hour watches under the 18-hour schedule. In order to combat hunger and mental fatigue, sailors are given the ability to take a break in order to regain their focus. With 16 hours between watches, sailors also have more free time. This gives them the opportunity to take care of their other shipboard duties, read a book, watch a movie, or exercise.
One small unforeseen problem - since sailors are on watch at the same time every day, the watch teams would be stuck with the same meals on each watch. To prevent this, meals were rotated every two weeks, so that watch teams got a bit of variety. Other submarines commanders have opted to instead shift watch times for the crew as a whole every two weeks.
So far, all the reports about the 24-hour watch cycle have reflected an improvement in both the submariners morale and performance.