GMEDX
BA/VS have a 48/72 hours down route between sectors.
CX do it 4 crew with a short stay, normally around 20 hours in the hotel before returning. 3 crew flights will miss a sleep during the sector, and then the time down route will be too short to recover from that missed sleep, let alone then next sleep.
Its an accountants dream, until all the crew start going sick, humans cannot sustain that, that is why BA/VS have the longer down route stay.