Why does travelling make you feel so tired?
There is lots of data on the "stressors of flight" (many of these stressors are also present in automobiles). As the vehicle moves, the sway and direction change cause you to have to keep yourself upright. These micro movements cause your muscles to be continuously working (even if you don't realize it). The US Air Force has done studies report conistent exposure to aircraft vibrations can lead to fatuige and increased chance of health problems.