Two trips cross the same number of zones. Why does the eastbound one usually bite harder?

Answer: Body clock must advance

The clock must delay — Delaying the clock is the westbound job, not the eastbound one. A delay means your body accepts a later sleep and wake time, which is usually easier because the average human clock runs slightly long. This wrong answer is close on purpose: it names a real jet-lag mechanism, but points it in the wrong direction.

Body clock must advance ✓ — 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.

It is just lost sleep — 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.