Last time I deployed on a ship (2011-12), the amount of time you had to sleep really depended on the job. Our engineering department on the ship, it was nothing for them to stay up 2 days straight. They had working hours, watch, drill, then maybe 3 hours and had a choice between hitting the rack or taking a shower and eating and chose the later. The IT department most of our deployment worked on 8-hour shifts and did not have watch. Just depends on the job and events of the day. I usually did a 12-on-12-off but I was also the main repairman for my division, so if repairs needed to be made, it ate into my "off shift time" while deployed.