Nor is doing 8-8-8 or 12-12 shifts. There are only so many people on a boat and they all need to be working about 50% of the time, plus all-hands stuff where nobody sleeps.
The risk of that logic is that 1-1 shifts would have 50% of the ship working at a time, although most of them would die after a week or so.
1-1 is clearly absurd. So is 2-2. And 3-3. I'm convinced 6-6 is also biologically unsustainable for most people, although 6 hours is long enough that 1) some people are perfectly fine with it, and 2) most other people can hack it for at least a while before incipient mental breakdown.
