Its pretty easy thing to do fairly. The few times that I've ridden I've suggested that we draw numbers. I write 1-2-3-4...on slips of paper folded and put them in my hat. I let everyone else draw. I get the remaining one. The high numbers get their choice of when they rest. We all split the time. If there's 4 of us and its a 6hr flight, we each get about 2 1/2 hr rest.
Next time I do it I have to ask the 2nd shift if they want to be woken to go to the bunk if they're asleep
Pere