Recovery calculator
Jet lag calculator
Get the days to adjust after a flight and the light schedule that decides how many — when to seek bright light and when to keep it dark, on the destination clock. Free, instant, no sign-up.
These windows describe the first day after you land. As your clock shifts, the minimum moves with it and the windows follow.
Travel wrecks a training week more reliably than the training does. The coach plans around the flight.
Why flying east is the half that ruins the week
Westward costs about a day per time zone and eastward about a day and a half, and the asymmetry is not a quirk of the itinerary. The free-running human circadian period is a little over 24 hours, so the clock drifts later on its own. Delaying it further — which is what flying west asks for — works with that drift. Advancing it fights the drift, which is why coming home from a holiday east is the trip that costs you the week.
Everything hangs off the temperature minimum
The core body temperature minimum is the pivot of the light phase response curve. Light in the hours before it pushes the clock later; light in the hours after it pulls the clock earlier. Get that backwards and you actively make the trip worse, which is the failure mode of "just get some sunshine when you land": landing in Europe at 7 a.m. off an overnight from the US means walking through arrivals into bright light an hour or two before your CBTmin, delaying a clock you are trying to advance.
Why the minimum is a band, not a minute
CBTmin cannot be measured without a lab, so it is estimated from habitual wake time. Two rules of thumb are in print — two hours before waking is the common one, three is the figure Burgess (2011) gives — so this page reports the whole two-to-three-hour band rather than pretending to the precision of a single minute. The true value moves with sleep duration and chronotype, and can only be pinned down by measuring melatonin onset. An hour of uncertainty is small against a four-hour light window, but it is real.
The light windows
To phase delay, seek light in at least the four hours before the minimum and avoid light in at least the four hours after it. To phase advance, the reverse (Burgess, "Using bright light and melatonin to reduce jet lag", 2011). "At least" is the operative phrase: a wider window works better, so four hours is a floor rather than a target. Avoiding light at the wrong time matters as much as getting it at the right one. The windows come back on the destination clock, because that is the clock you can read while your body is still keeping home time.
Two different numbers called days
The rule of thumb tracks how long the symptoms linger and is what every travel guide quotes. The other two track the clock itself, at rates measured in travellers: without managing light, people delay up to 1.5 hours a day and advance up to 1 hour a day; with light sought and avoided on schedule, about 2 hours a day delaying and about 1.5 advancing. The gap between those two figures is the entire argument for bothering with a light plan.