I fly LAX - SIN on UA quite often, always could get used to the time different / jetlag on the LAX - SIN as arriving around midnight, so could just sleep, even for 5 to 6 hours, that's enough. But on the SIN - LAX, UA's flight arrive in the morning, or early afternoon and there's still a big half of the day to stay awake before the actual sleeping time, end up sleeping very early (like 7pm) always.

As for easier on body, do you mean the UV radiation thing? I saw a post some time ago saying that UV (or whatever rays that are harmful to the body) is more intense near the poles, so the polar flights are less easy on the body?

ps. I wont heed the UV theory anyway.