East is harder. The body clock adjusts about an hour a day flying east and an hour and a half a day flying west, because left alone it drifts later rather than earlier. London to Tokyo is nine hours east: expect more than a week to settle, though you will be functional by day four or five. New York to Paris is six: most of a week. Tokyo back to London is west, so it is quicker, but the first few days still drag.
Arriving in Asia from Europe or the US. Stay up until a local bedtime of 9 p.m. or 10 p.m., outdoors as much as you can: afternoon and evening daylight on the first day tells the clock to move. If you are falling over by 4 p.m., take a twenty-minute nap sitting up, not in a bed. Curtains shut at 3 p.m. is how people lose a day.
Flying home west. The problem reverses: you wake at 4 a.m. and fade by 8 p.m. Avoid bright light early in the morning for the first couple of days, seek it in the late afternoon and evening, and push bedtime later by an hour a night. Westward usually fixes itself.
Caffeine. Coffee in the morning and early afternoon at the destination keeps you upright and does not fight the shift. A cup or two in daylight is fine. Stop six hours before the bedtime you are aiming for. Alcohol on the plane makes the first night worse.
Melatonin. The evidence is reasonable for eastward travel: 0.5 to 1 mg at local bedtime for the first three or four nights. The caveats: it is sold as a supplement in the US, so the amount in the pill may not match the label; it needs a prescription in the UK and much of Europe; and taken in the middle of the night, when the body is already making its own, it does nothing. It moves the clock a little; it is not a sleeping pill.
Set the trip up for it. Day one is walking, outdoors, no fixed times: a park, a neighborhood, a market, a slow lunch, an early dinner. No timed tickets, no day trip, no table booked for 8:30 p.m. Put the timed entries, the early trains and the long day trips on day three or four. Choose an afternoon arrival for an eastbound flight where you can; morning landings hurt most. Build day one as a loop from the hotel; if Waystop’s travel times show more than ten kilometers of walking, cut it.
On the day. Land, shower, eat, walk in daylight, nap for twenty minutes at most, eat again at the local hour, and go to bed at 9:30 p.m. with the curtains open a crack so the morning light does the rest.