For what it’s worth I did find that I adapted to long flights and jet lag. The first time I did the NYC to LON redeye I was a mess the first day. After doing it every other week for awhile it was no problem. Then I did a Dallas to Sydney flight and found I had no trouble. I moved my sleep around a couple days before so that I would sleep for the middle 8 hours of the 16 hour flight. Being awake for four hours on either side of a night of sleep and getting up to walk around a lot made it a breeze.