I was discussing with a flight attendant supervisor why air travel was so exhausting, even though we're just reading, napping, or eating while in flight. After covering the usual stress of delayed flights, cramped seats, and lack of fresh air, he said something I'd not heard before:
Airlines tell new flight attendants that, in terms of wear and tear on the body, one hour in the air is equal to three hours on the ground. The pressurized cabins squeeze our bodies, even though we can't consciously feel it. I don't know if this pans out scientifically, but it made sense to me. No wonder we can be tired after a cross-continent flight.
The noise factor on airplanes is much more important than we believe. Planes are very noisy. After a while, our conscious mind filters out the noise and we are no longer aware of how noisy it is. But our subconscious mind is still taking a beating. I've found that the simple act of using inexpensive ear plugs cuts the stress of air travel in half.