East is harder, and there is a reason
The rule of thumb is about one time zone a day travelling west and roughly one and a half days per zone travelling east. That asymmetry is not folklore. The human circadian clock free-runs at somewhere between 23.8 and 24.6 hours, and for most people it is slightly longer than 24.
Travelling west asks the clock to delay — to stretch the day — which is the direction it already drifts. Travelling east asks it to advance, which fights the drift. That is all there is to it, and it is why a six-hour shift feels different depending which way you flew.
The hinge is two hours before you wake
Your core body temperature minimum sits roughly two hours before your habitual wake time, and it is the pivot the entire schedule turns on. Light in the six hours after it shifts your clock earlier. Light in the six hours before it shifts your clock later.
Get that backwards — and it is easy to, because "get morning light" is the advice everyone half-remembers — and you move the wrong way. If you normally wake at seven, that hinge is around five in the morning. Light at four is delaying you; light at eight is advancing you.
Shifting a couple of days before departure is the highest-value thing most people can do, and almost nothing that ranks for this query mentions it. Thirty to sixty minutes a day over three or four days takes a meaningful bite out of the adjustment before you have left home.
Eight zones east is a different problem
Past about eight time zones eastward the body clock frequently re-entrains the wrong way, drifting later instead of earlier, because evening light at the destination gets misread as morning light. The literature calls this antidromic re-entrainment.
So a naive "get up an hour earlier each day" plan can actively make a long eastward trip worse. On those trips it is often better to treat the shift as a westward one and be strict about avoiding light in the local morning for the first day or two.
No consumer jet lag calculator I could find models this at all, which is a little surprising given that London to Tokyo and New York to Delhi both sit well past the threshold.
On melatonin
The CDC says 0.5 to 1 mg is often enough to produce a circadian shift and does not recommend doses above 5 mg. A 2019 review in Frontiers in Physiology favours 3 mg. That is a genuine disagreement between reputable sources rather than a settled number, so it is presented here as a range.
What both agree on is timing: taken at the wrong point in your body clock — roughly midnight to five a.m. in body time — it does nothing useful. The dose is less important than the hour.
Things people ask
How many days will it take?
Roughly one day per time zone westward, and about one and a half eastward. A six-hour westward shift is usually most of a week; the same shift eastward can be nine or ten days.
Is it worth adjusting before I go?
For anything over three or four zones, yes. Thirty to sixty minutes a day for three or four days is realistic and removes a meaningful part of the shift before departure.
Is a short trip worth adjusting for?
Often not. If you are away for two or three nights, staying on home time and scheduling around it usually beats a half-finished adjustment in both directions.