Jet lag has a direction, and that changes what works

Most explanations of the body clock on arrival stop at the point where it starts to matter. This one carries on.

Why east is worse

The human circadian rhythm runs slightly longer than twenty-four hours, so delaying it is easier than advancing it. Flying west lengthens your day, which the body accommodates readily; flying east shortens it, which it resists. This is why the return leg of a trip is often much harder than the outbound one, and it is not imagination.

Practically, expect eastward adjustment to take noticeably longer for the same number of zones.

Light is the main lever and the timing is directional

Light in the morning at your destination advances the clock, which is what you want after flying east. Light in the evening delays it, which is what you want after flying west.

In practice, getting this backwards actively slows adjustment, which is how people spend a week feeling wrong. The critical window is the few hours around your body clock's temperature minimum, roughly two hours before your usual waking time.

Adjust before you go, a little

Shifting your sleep by an hour a day for two or three days before departure reduces the gap you have to close on arrival. Even a partial shift helps, and a full one is rarely practical.

For short trips of two or three days, staying on home time is frequently the better strategy than adjusting twice. Moving mealtimes along with the sleep shift appears to help, since digestion runs on a clock of its own that light does not reset directly, though the evidence here is thinner than it is for light.

On the aircraft

Setting your watch to destination time on boarding is a useful psychological anchor and costs nothing. Sleeping when it is night at the destination, and staying awake when it is not, is the substance of the advice. Alcohol fragments sleep and dehydrates, and the cabin environment already does the second of those.

Drinking water and getting up to walk are worth doing for comfort and circulation, and neither shifts a body clock, which is the claim most often attached to them.

Melatonin and medication

Melatonin is used to shift circadian timing and its effect depends heavily on when it is taken relative to your own rhythm. Availability and regulation vary widely by country, and dosing advice differs between sources. It is worth discussing with a pharmacist or doctor rather than guessing, particularly alongside other medication.

On a long trip, sleeping tablets do something different again: they can produce sleep at the wrong time without moving the underlying rhythm, which is why the following night frequently goes just as badly.

The first day sets the pattern

Staying awake until a local bedtime on arrival, even a slightly early one, is the highest-value thing you can do. A nap longer than about twenty minutes on the first afternoon reliably undoes it. Outdoor daylight and physical activity on day one accelerate adjustment more than any supplement.

The working rule: if staying upright is genuinely not happening, a short nap taken early in the afternoon with an alarm set costs far less than the same nap taken at six in the evening.

The takeaway

Work out which direction you flew, then get the light timing right. That is most of it.

Every kilo you leave behind pays you back at the third connection.