According to research, at 7+ hours of sleep, a soldier is 98% combat effective.
At 6 hours, that drops to 50%.
At 5 hours, the soldier’s down to 28% combat effectiveness.
4 hours or less, and the soldier is practically a liability at only 15% combat effectiveness.
Before the most recent research, the US Army said that 4 hours of sleep was considered "adequate".
Soldiers need to take "tactical naps."
Enter DRDC's sleep research for the Navy and different watch rotations. They have a pile of data.
On my OP REASURANCE we went to a modified watch rotation. Divide the day into 4-hour blocks. Then assign to each sailor 8 hours for sleep, and then an adjoining 4-hours where they were "available" but not on watch. Availability meant they could do whatever they liked (sleep more, gym, video games, reading etc...) but had to show up for evolutions (like a RAS). The Master Sailors were empowered to protect their sailors off time so they were not called in for random evolutions that could be done by the on-watch persons and helped to ensure that the watches were organized with enough people to ensure that routine evolutions didn't need the extra help. Basically, each sailor had 12 hours "off watch" and 8 hours "on watch" with a 4 hour break in the middle.
There were no routine pipes. No wakey wakey, no hands to supper etc... people knew what time it was and showed up to eat, work, on watch at the proper time because they are adults. Food was served at the same times but there was a midnight meal laid on of sandwiches/leftovers for overnight watches. This had an odd side effect of reducing the amount of food eaten on board as people grabbed lighter meals (particularly breakfast).
The dayworkers ran normal working hours but supervisors were empowered to set whatever hours, and staffing they saw fit. Like professional adults they were. And not surprisingly they set office hours that made sense for the new watch rotation.
For my day I naturally woke up around 730ish, went to the wardroom for ~8, grabbed toast/cereal, and made a tea. Chilled for 30min, and then went to work. I worked till about supper, and then went back around 7 and worked some more till about 10. Except on Thurs which was Movie Night or boardgame night.
Of course, this changed when we were in a higher readiness state, and switching between states provided a challenge but we figured it out.
I've never been so rested and productive at sea in my life, ever. It took a little getting used to but the focus on sleep, and rest really paid off I think.