The time changes alone in that kind of travel can put you out of sorts for multiple days after you reach the destination.
Last week I flew to Malaysia from the US East Coast, and my total transit time was around 26 hours, door-to-door, including one layover and two car rides of multiple hours each. By the time I got to Malaysia I was whipped, as I always am on that trip, and for the next four days it was “work the day; get to the hotel at around 6:30 PM local time; shower and sleep for six to eight hours”. So I’d be waking at around 4 AM and starting my day then. (The time change between Malaysia and US Eastern Daylight Time is exactly 12 hours.)
Now that I’ve finally gotten acclimated to the time… it’s time to travel again. This week I’m in India, but fortunately the time difference is only 1.5 hours.
Flying “towards the East” always is harder on me, too, but that could be just because it’s the outbound trip for me. Returning to my home I’ll be back on US Eastern Daylight Time within two days, guaranteed.
And getting sick is easy, regardless of “bad cabin air” (which I do not subscribe to; I recall the days when smoking was permitted – and common! – on airplanes; the air is much better now), because of the various stresses that travel itself places on you. Not to mention the stresses of having to meet rigid travel schedules. My day-to-day routine requires that I be ontime for work, for example, and I do have meeting schedules to adhere to during the day and week, but that’s not the same as “miss the time, and you’ll miss the connection”. That involves a lot more stress for me.