system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do I feel so awful after a 4 hour flight, but feel fine after 4 hours at my desk?
gpt
A lot of reason,s actually. In an airplane, the pressure is lower than you're likely used to (in most, it's normalized at the pressure roughly equivalent to ~7,000 feet above sea level). Unless you live that high up (and there are only 14 states that even *have* peaks much above 7k feet) then you're going to feel a difference, and you ascend to that pressure so fast that you don't have much time to adjust. So there's some just basic pressure shock involved. 

Airplane air is also generally *very* low humidity, and that can dry you out and require additional water intake to offset, which most people don't get on the plane. It's recirculated, too, so any bad smells will often find their way back to you throughout the flight, albeit at lower levels. Then there's just the additional stress of travel -- crowds, security, etc.