Why Jet Lag Hits Harder Flying East Than Flying West
The human circadian clock's natural cycle tends to run slightly longer than exactly 24 hours, which makes it naturally easier to adjust to a longer day (as happens when traveling west, effectively gaining time) than to a shorter day (as happens when traveling east, effectively losing time) — this is why eastward travel commonly produces more severe or longer-lasting jet lag than an equivalent time-zone shift traveling west.
This is a genuine, well-documented physiological asymmetry, not simply a matter of perception or trip-specific circumstances.
The natural circadian rhythm length
Without external time cues, many people's natural internal clock cycle runs somewhat longer than exactly 24 hours — this built-in tendency toward a slightly longer natural cycle means the body finds it easier to adapt to a day that's effectively lengthened (westward travel, where local time falls behind compared to the body's internal clock) than one that's effectively shortened (eastward travel).
Why eastward travel requires the harder adjustment
Flying east requires the internal clock to shift earlier — going to sleep and waking up earlier than the body is naturally inclined to, working against that natural tendency toward a longer cycle — which is mechanically a harder adjustment than the westward direction's effectively longer day.
What this means practically
For an eastward trip, starting the adjustment process earlier and being more deliberate about light exposure timing (as covered in the adjustment-plan guide) is especially valuable, given that this direction works against the body's natural tendency rather than with it, unlike westward travel.