Jet Lag Calculator

Roughly a day per time zone travelling east, and a day per one and a half zones travelling west. Nine zones east, London to Tokyo, takes most people about nine days to fully adjust.

The human circadian period is slightly longer than 24 hours, so the body finds lengthening a day easier than shortening one. Flying west means staying up later, which most people manage; flying east means sleeping earlier, which most people cannot.

Timed light exposure, by a wide margin. Going east, seek morning light and avoid it in the evening; going west, do the reverse. Getting the timing backwards actively delays adjustment rather than doing nothing.

For a long eastward trip, moving your bedtime earlier by an hour a day for a few days beforehand meaningfully reduces the shift left to make on arrival. For short trips it is usually more disruption than it is worth.

The evidence is reasonably good for eastward travel, taken in the destination evening. Timing matters more than dose, and taking it at the wrong hour can shift your clock the wrong way — worth reading properly or asking a pharmacist before relying on it.