When I was in, we had a 6-on 12-off schedule to allow for 24hr coverage with only three watches. Effectively, each of our "days" was an 18hr day, with 6 being on-watch, 6 being used for maintenance/drills/training, and 6 being for sleep. Rarely did we get a full 6hrs of sleep per 18hr "day."
That sounds like the worst possible compromise between 16+8, which gives you a normal "on" work shift and a normal sleep period, and 8+4, which could give you biphasic sleep, or triphasic with 4 hours base sleep and 2 x 2-hour naps.
Even so, it's still 8 hours of sleep per 24 hours, which is adequate, even when only being able to take 6 at a time makes it somehow seem worse. And I have a sneaking suspicion that it only exists because the Navy found that sailors were useless after the 12th hour of working, so decided to set the workday to 12 hours, and just make the whole duty cycle 18 hours. I'm guessing that people on that schedule stop being very effective after just 9 hours of wakefulness. But I'm no somnologist.
The 12-on is composed of a 6-hour operator shift, followed by 6 hours of maintenance and training. I think the 12+6 is based on maintaining full awareness over a 6 hour period, and in that sense it basically works. If you eat when you wake up, and after your operator shift, then that also works out to two meals per 18-hour day, and gives you a normal-ish 12 hour gap between your last meal and waking up.
That is correct. The 6 hour on-watch period is based on maintaining full awareness. With a 3 watch-section crew and "18hr days," you'll have round the clock coverage for all watch stations. Like you said, we'd wake up, eat, go on watch for 6hrs, get relieved by the next watch section, eat, do maintenance/training/etc. for the next 6hrs, and then sleep for the final 3rd of our "day."
