Jet lag calculator

What the number means

Jet lag is a mismatch between your internal clock and the local time after you cross several time zones quickly. The CDC Yellow Book describes it as temporary desynchronization between the biological clock and local time, with symptoms that include disturbed sleep, daytime sleepiness, impaired thinking, malaise and stomach upset. The number this calculator gives is not a measure of how bad you will feel. It is the average time the clock itself needs to move by the number of hours you crossed.

The method

First the calculator subtracts the two UTC offsets. A difference of +9 hours means the destination is nine hours ahead, so your clock must move earlier (eastward travel); a difference of −5 means it must move later (westward). A difference larger than 12 hours is read the short way round the clock: Tokyo to New York is a 14-hour difference, which the calculator treats as a 10-hour shift east, about 10 days. Then it divides the shift by the CDC rate: 1 hour per day going east, 1.5 hours per day going west. Days = hours to shift ÷ rate. The division is our own arithmetic on the CDC averages.

If you choose to shift your sleep before departure, the calculator removes up to one hour per day (at most three days) from the shift to be made after landing, and prints the bedtime for each day before the flight. The CDC words this carefully: shifting sleep one hour per day for two to three days "may reduce" the time needed to adjust.

Worked example: Los Angeles to London

Los Angeles is on UTC−8 in winter and London on UTC+0, so London is 8 hours ahead and the clock must move 8 hours earlier. At 1 hour per day that is 8 days. If you go to bed 1, 2 and 3 hours earlier on the three nights before leaving, the shift left to make on arrival is 5 hours, so about 5 days. The way home is the same 8 hours the other way, but the shift is later, at 1.5 hours per day: 5.3 days, or 6 when you count whole days. That asymmetry is what the CDC means when it calls westward travel easier to adapt to.

Limits of this method

The CDC rates are averages across travelers, not a prediction for you. The Yellow Book adds that sleep lost during travel can worsen symptoms, that the best timing of light exposure depends on the direction and the number of zones crossed, and that bright light in the morning after the body's low point moves the clock earlier while evening light moves it later. It suggests choosing a flight that arrives at a time giving the right light exposure. The calculator cannot plan that for you.

Second, the offsets. Time zone offsets change with daylight saving time, and a few places keep half-hour or 45-minute offsets, which the list includes. Check the offsets on your travel dates. Third, the CDC criterion for jet lag disorder is travel across at least two time zones, so one-hour differences are outside what its tables describe. Fourth, a 12-hour difference or more gives no clear direction, and body clocks do not always follow the shorter path. For trips shorter than about two days the CDC suggests staying on home time instead of adapting.

The Yellow Book also recommends staying hydrated, avoiding excess alcohol, minimizing caffeine near bedtime and avoiding sedative medications with long half-lives. Planning a trip involves other numbers too: see the flight distance calculator for the flight length, the sleep calculator for bedtimes in whole 90-minute cycles and the time zone converter for meeting times.

Frequently asked questions

How long does jet lag last?

The CDC Yellow Book gives average rates for the body clock to catch up: about 1.5 hours per day after westward travel and about 1 hour per day after eastward travel. Divide the hours you crossed by that rate and you get the days. Flying from New York to London (5 hours east) comes to 5 days; the return trip west takes 3.3 days on the same arithmetic. These are averages for the clock itself; how tired you feel on a given day, and how fast you adapt, varies from person to person.

Is the rule really one day per time zone?

For eastward travel the arithmetic gives that: 1 hour of clock shift per day means one day for each zone crossed. The CDC does not state a day-per-zone rule, only the rates of 1 hour per day east and 1.5 hours per day west. Going west the same zones take two thirds of the time. A 5-zone trip west is 3.3 days, not 5.

Why is flying east worse than flying west?

The CDC explains that the average period of the human body clock is slightly longer than 24 hours, so it is easier to delay it than to advance it. People naturally want to go to bed a bit later each night even without travel. Heading west asks the clock to run later, which suits it; heading east asks it to run earlier, which does not.

Can I prepare before I leave?

The CDC says that shifting sleep by 1 hour per day, later for westward and earlier for eastward travel, during the 2 to 3 days before the trip may reduce the time needed to adjust. The calculator subtracts those hours from the shift still to be made after landing: Los Angeles to London is 8 hours east, 8 days on the average rate, and 5 days if you moved your sleep earlier by 3 hours first. The schedule in the results shows your bedtime for each day before departure.

What if I'm only staying a day or two?

For short trips of less than about two days the CDC advises not trying to adapt: keep your sleep, activity and meal times on your home schedule. The calculator shows a note when you enter a stay shorter than two days.

Which direction does the calculator use when the difference is more than 12 hours?

It follows the shorter way round the clock by default. Tokyo to New York is a 10-hour shift forward or a 14-hour shift back, so it shows 10 hours east, about 10 days. Delhi to New York is 13.5 hours east or 10.5 hours west, so it shows the westward 7 days. You can force the other direction with the direction list; individual body clocks do not always take the shortest path.

Does the calculator say anything about melatonin or sleeping pills?

No. This page only does the arithmetic of the published average rates and the bedtime shift. The CDC chapter also covers timed light exposure, melatonin and other medications; whether any of that is right for you is a question for your doctor or a sleep specialist, especially if you take other medicines or have a sleep disorder.

General information only, not medical advice and not a substitute for a doctor or sleep specialist. Sources: Riedy SM, Williams SG, “Jet Lag Disorder,” CDC Yellow Book 2026, Chapter 7 (Travel by Air, Land & Sea), cdc.gov/yellow-book/hcp/travel-air-sea/jet-lag-disorder.html (page dated April 23, 2025): the criterion of at least two time zones, 1.5 hours per day westward and 1 hour per day eastward, shifting sleep 1 hour per day 2 to 3 days before the trip, and the advice for trips under two days. Days to adjust are our arithmetic on those averages. How we check our calculators.