The formula
days ≈ zones ÷ rate · about 0.66 h a day eastwards and 1.5 westwards
Where it comes from
The body clock does not reset all at once: it shifts a little each day, and not equally in both directions. Flying west lengthens the day, which the body handles well because its natural cycle runs slightly longer than twenty-four hours: it recovers around an hour and a half a day. Flying east means shortening the day, which works against that tendency, and the rate drops to about two thirds of an hour a day. That is why the same six-zone hop is four days coming back from New York to Madrid and nine days going out.
How to work it out by hand
- Count the time zones between origin and destination
- Pick the rate for the direction: 1.5 h a day westwards and 0.66 eastwards
- Divide the zones by the rate
- Take off whatever you shifted your clock in the days beforehand
What is worth knowing
Light is what resets the clock, and it works in both directions if used at the right moment: morning light advances the body clock, which is what you need flying east, and evening light delays it, which is what you need flying west. Shifting your bedtime by half an hour a day for the two or three days before you go reduces the lag left when you arrive, and it is the only thing with solid evidence behind it besides light. Eating at destination times from the first day helps; sleeping all afternoon on arrival day does not.