Why does flying east across time zones usually feel harder than flying west?

Answer: Your body clock has to shift earlier

Your body clock has to shift earlier ✓ — Correct! Eastbound travel asks your circadian clock to advance: sleep and wake should happen earlier than your body expects. For most people that is harder than delaying the clock, because the human clock naturally runs a little longer than 24 hours. The memorable part is that a route can feel different simply because the body is being asked to move time in the harder direction.

Your body clock has to shift later — Wrong. Shifting later is what westbound travel asks for, and it is usually easier because the average human clock runs slightly long. This option names a real jet-lag mechanism but points it in the wrong direction.

You simply lose more sleep on eastbound flights — Wrong. Lost sleep can worsen any trip, so this is a tempting explanation. But sleep loss alone does not explain why east and west feel different when the clock shift is the main problem. The useful surprise is that east and west are not mirror-image inconveniences, because one compresses the body day while the other lets it stretch.