This is why flying on a plane makes you feel terrible

Do you feel inexplicably crappy — tired, dehydrated, and headachy — every time you fly?
 It's not your imagination. People talk a lot about the many awful aspects of flying nowadays, but one that gets less attention is the way that sitting in a small, pressurized metal tube at 35,000 feet for several hours wreaks havoc on your body. "Anytime you fly, you're exposing yourself to a different environment than your body is used to," says Jeffrey Sventek, director of the Aerospace Medical Association and a longtime aerospace physiologist for the Air Force. For some people, this environment — with lower oxygen levels than the ground, extremely little humidity, and sudden changes in air pressure — can cause a bunch of negative symptoms.
 This is how flying can make you feel terrible.