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.