Arriving in a distant time zone leaves the body running on the departure schedule for some days afterwards. The adjustment follows a predictable pattern that direction of travel strongly influences.
The internal clock runs slightly long
Human circadian rhythm has a natural period a little longer than a day, and it is corrected continuously by external cues, chiefly light.
Because the underlying tendency is to drift later, pushing the internal day back is easier than pulling it forward.
This asymmetry is why westbound travel, which lengthens the first day, generally settles faster than eastbound travel, which shortens it.
Light timing is the strongest signal
Exposure to bright light shifts the body clock, and whether it shifts earlier or later depends on when in the internal cycle the exposure occurs.
Light in the internal morning tends to advance the clock, while light in the internal evening tends to delay it, which means timing matters more than duration.
The practical consequence is that getting outside at the right point in the destination's day does more than any amount of staying indoors and resting.
Meals and activity reinforce or fight the shift
Digestive and metabolic rhythms respond to when food is eaten, and they adjust on a somewhat separate schedule from the sleep cycle.
Eating on the destination's schedule from the first day gives a consistent signal, whereas eating on the departure schedule prolongs the mismatch.
Physical activity in daylight supports the same adjustment, which is one reason arrival days spent walking tend to end with better sleep than those spent in a room.
The first night is rarely representative
Sleep on the first night is often adequate through sheer accumulated tiredness, and the difficulty appears on the second or third night instead.
This surprises travellers who conclude after one good night that they have adjusted, and then plan demanding activity for the point where the effect is strongest.
Scheduling the least demanding part of a trip for days two and three fits the pattern better than front-loading recovery time.
Short trips may not be worth adjusting for
For a stay of two or three days across several zones, the body will barely adjust before the return, and attempting to force it can leave the traveller unsettled in both places.
Staying partly on home time is a recognised approach for short business trips, with meetings arranged where possible around the hours that overlap.
Anyone with a medical condition affected by sleep or medication timing should raise it with a doctor before travelling, since general scheduling advice does not address those cases.