Right. The biggest circadian concern is consistency. It takes about a day to fully adjust your circadian cycles by an hour. You can shorten that somewhat by using certain lighting and possibly supplements like melatonin, but you can't just flip back and forth like our schedules demand we do. The mathematical reality on that is quite brutal to front/back side of the clock operations which is why we'll never see true circadian protections written into the regs. They invented the notion that you can "acclimate" to the other side of the world in 30-something hours, which is completely bogus. OTOH, we'll never get full circadian protections in any reg or contract because that would essentially eliminate international flying without a massive increase in manpower.

There is nothing unsafe about CDO trips compared to other flying airlines do, provided pilots stay on that cycle. If that was the case, they would in many cases be more safe than the flip flopping we do both domestically and internationally. But I'd bet the company would want no part of a fixed CDO line schedule with adequate protections for reserves though because that would cost them more than it would save them. The main reason they'd want them would be efficiency and flexibility to sprinkle them out in bid packs and open time and let the coverage fall where it may. That would degrade safety for sure.

As for what they'd save by having them, there's some built in savings from hotel and per diem cost, but in most cases the credit time would be extremely high compared to the block time. IMO they don't want them enough to pay the cost of having them.