PPS: Keg - I hesitate to say this, and I mean NO disrespect at all, but you have 'only' been Blunderbussing for a short while.
This is true. I do know that I found jet lag the second time around on the 744 much easier than the first. The difference? Apart from a couple of very big nights (one with Going Boeing that he remembers far more of than I do), I looked after myself much more effectively the second time around. Still, I guess I'll know more if I'm still on the Bus in a couple of years. (Though truthfully I'd prefer to be heading to the 787).
What I am alluding to is very long term tiredness. And it's a personal thing, I don't like it and a lot of other people I know don't either, but others seem to deal with it quite well. A study would be interesting......
A study would indeed be interesting and there is a PhD there for the keen. It'd need to be pretty in depth and include such things as food, exercise, and sleep diaries.