Most Sailors on ships work in watch rotations. If you’re lucky it’s three shifts, but more often than not it’s “port and starboard” shifts of 12-on/12-off. This is especially hard for those who work nights.

Even if your shift ends at, say, 0800, you still have to eat, then attend muster (like a daily standup), then do an hour or so of “cleaning stations”. Afterward you can’t go to sleep yet because the berthing where you sleep is waiting for the XO to inspect. Once that’s all done, you are still up because they planned a General Quarters (GQ) drill or a man overboard at 1000. Two hours later and you are finally free, but it’s lunch so you might as well eat. You have about 8 hours left, but you are also delinquent on 2 or 3 qualifications that require walk-through a or attending trainings which only happen during the day. Maybe you want to send an email home, so you go to the library to wait for a free computer.

By the time that’s all done, you have about 4 or 5 hours to sleep. You might be able to do that without problem for a day or two, but a week later and you are friend.

Or if your on an LHD like I was, Marines get bored and toss a chem light in the water and make everyone wake up for man overboard.

good description of the practices, but it doesn't answer the question, which was _why_ they arrange the schedules and duties that way

They actually have training / excercises within the Army (and I assume other branches), where they intentionally sleep deprive you while evaluating you on tasks. It’s assumed there will be points in battle where you can’t sleep. 24hrs without sleep happened fairly often, but the longest training that I can recall with sleep deprivation lasted 72 hours. Most people did manage to “sleep” during that time, but 20 minutes at most, as the cadence of the tasks was designed that you couldn’t sleep at all. I must admit though my platoon could mostly do our jobs in our sleep because the actions had been drilled into us and we had done them so many times with little or no sleep.

There's a significant difference between training to handle sleep deprivation as part of specific operatipns and just being sleep deprived 24/7 for months or years, though, and the US military has a long history of doing the latter.

Sleep deprivation might be a form of trauma bonding honestly.

Another word for hazing?

Poor time management, there's a lot of "hurry up and wait" and "if you're not 15 mins early, you're not on time" in the military.

The 15 minute early thing compounds as well: each level of supervision expects their people to show 15min early for a variety of reasons; at the lowest levels it's to take roll and address absences ("send someone to go wake up Smith, NOW!") and to brief individual or small group* responsibilities abd/or meet up points, and might even include travel to an intermediary or ultimate rallying point which itself has a 15 minute early policy. And from there it continues. 15 minutes for each subgroup to arrive at and find their place in the bigger group, repeat until everyone is gathered. Could end up being hours early as an individual in order to account for this. In the end it's nothing really important and your day was "wasted" (cue separate discussion regarding "your" time in a military).

* group meaning a collection of people, not the occasionally-used military "Group"

Relevant comic: https://pics.me.me/broken-and-unreadable-william-melancon-15...

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.

3 additional man hours daily either reduces Manpower needs by 15-20% or makes them more productive. A marginal hour of a soldiers time is free.