I’ve long wondered whether the concept of Time Dilation has anything to do with that very specific type of tiredness one feels after a flight.
The attempt by your body or brain to account for and remedy the minuscule but measurable time lost after the high velocity travel may well fatigue it in a unique way.
Its noticeable enough as an inactive passenger, but must be worse for the flight crew who have to work as well.
I can’t find any research or mention of the effects of Time Dilation on ISS crew or astronauts. I suppose if it was a problem the effects would be felt far more severely by them than airline pilots?