doulos05 is right. They have four hour shifts.
When I was in the Navy we had four duty section in port and three at sea.
Only one duty section had watch standing per day. Everyone else had their normal duties to attend to. Each section was also split into three subsections. We had four hour shifts so if you went on at midnight (first watch) you had your shift till 0400. At 0730 you mustered for the days normal duties and at 1200 went back on watch again.
Each subsection had to stand watch twice a day for their time zone.
For the record I hated the second watch (0400-0800 and 1600-2000) shift. It made for a long day.
