Jet lag calculator: adjustment days (estimate) and light timing

How does the jet lag calculator work?

After a long flight your body clock still runs on home time. Light in the hours before your body’s nightly low point moves the clock later, light in the hours after it moves the clock earlier, and the largest shifts come in the 3 to 6 hours on either side (Roach & Sargent, 2019).

The calculator estimates that low point from your usual wake-up time: someone who sleeps from 11 pm to 7 am has it around 4 am (Roach & Sargent, 2019). It then shifts the low point by the time difference and moves it back toward home time each day at the average rate.

How long does jet lag last?

It usually gets better within a few days (NHS, 2023). As a rough average, the body clock shifts about 1.5 hours a day after flying west and 1 hour a day after flying east (CDC Yellow Book, 2025), so six time zones east can take close to a week.

Why does the plan differ for east and west?

Flying west, your clock has to run later, so the calculator puts daylight in the hours before the low point, which usually means afternoon and evening. Flying east, it has to run earlier, so daylight goes after the low point, and light before it is better avoided (Roach & Sargent, 2019).

Soaking up daylight at any time of day after a long flight east can send mixed signals and slow you down (Roach & Sargent, 2019).

What this calculator is not

It covers the timing of light only, not medicines or supplements. Light plans for jet lag rest mainly on lab studies. Field studies with real travelers are few, and their results are mixed (Roach & Sargent, 2019). If you stay tired for a long time after a trip, talk to a doctor.