Every time I take an airplane, I feel tired and worn out. I don't want to do anything for the rest of the day except take a nap.

Trains are no problem.

The main reasons might be relatively low oxygen at altitude (cabins are usually pressurized somewhere between 1/3rd to 1/2 atmosphere) and uncomfortable seats. But I think the whole dreary process of getting on and off the airplane is part of it, too. Train stations are so much more low key, even in large metropolitan areas.