How many days your body clock needs after a flight east or west, a sleep plan for the nights before and after, and when light, caffeine and melatonin help, per the CDC.
Jet Lag Calculator - How Long It Lasts and When to Sleep
How many days your body clock needs after a flight east or west, a sleep plan for the nights before and after, and when light, caffeine and melatonin help, per the CDC.
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How long jet lag lasts, and a plan to shorten it
Type the time difference, the direction you fly, your usual bedtime and wake time, and how many days before the flight you can move your sleep. The jet lag calculator estimates how many days your body clock needs after landing, lays out the nights before and after the trip, and tells you when to seek light, when to avoid it, when caffeine helps and when to stop, all built on the rates and timing in the CDC Yellow Book chapter on jet lag disorder.
Four answers the form needs
- Time difference - hours between home and the destination, from 1 to 12, in half hours for places such as India (5.5 or 10.5).
- Direction - east when the destination clock is ahead of yours, west when it is behind.
- Usual bedtime and wake time - your normal schedule at home, in 24-hour time, for example 23:00 and 07:00.
- Days to shift before the flight - none, 1, 2 or 3. Each day moves your sleep 1 hour toward the destination.
Problem: the clock on the wall moves in one night, the body clock does not
After a 6-hour flight east, your watch says 11:00 PM while your body is still on 5:00 PM. Sleep will not come, and the next morning your body expects the middle of the night. CDC describes jet lag disorder as a mismatch after crossing at least 2 time zones, with symptoms starting within 1 to 2 days: poor sleep, daytime sleepiness, trouble concentrating, an upset stomach.
Solution: move the body clock on purpose, with sleep, light and timing
The body clock adjusts on its own at about 1 hour a day after flying east and about 1.5 hours a day after flying west, according to the CDC Yellow Book. You can start before you leave by moving sleep 1 hour a day for 2 to 3 days, and speed the rest with well-timed light. The calculator subtracts the hours shifted at home and divides what is left by the daily rate.
Below is what the calculator returns for some common routes with an 11:00 PM bedtime. The days are estimates for an average traveler, rounded up to whole days.
| Trip | Shift | No preparation | 3 days shifted first |
|---|---|---|---|
| Los Angeles to New York | 3 h east | 3 days | none |
| New York to London | 5 h east | 5 days | 2 days |
| New York to Paris | 6 h east | 6 days | 3 days |
| Paris to New York | 6 h west | 4 days | 2 days |
| Chicago to Delhi | 10.5 h east | 11 days | 8 days |
| Paris to Los Angeles | 9 h west | 6 days | 4 days |
Time differences change twice a year where clocks switch to and from daylight saving time, and not on the same dates everywhere. Check the difference for your travel dates before you type it.
East or west decides what you do with light
Bright morning light moves the body clock earlier and evening light moves it later, the CDC chapter explains, which is why the advice flips with the direction of travel.
| Flying east | Flying west | |
|---|---|---|
| Before the flight | bed and wake 1 h earlier per day | bed and wake 1 h later per day |
| Light | bright light early in the morning, none in the evening | no bright light early in the morning |
| Caffeine | in the early morning | not in the early morning |
| Melatonin | 0.5-1 mg, 90 minutes before bed | not part of the CDC westward plan |
| Adjustment rate | about 1 h per day | about 1.5 h per day |
The rest is the same both ways. Naps of 20 to 30 minutes help you through the afternoon; longer ones steal from the night. Caffeine at up to 200 mg every 4 hours in daylight keeps you alert, as long as the last cup is 6 hours before bed. CDC advises against high doses of melatonin, above 5 mg.
What the day-by-day table shows
Each row is one night. The home rows show the earlier or later bed and wake times while you prepare, in home time. From the arrival day on, bed and wake are your usual hours in local time, and the last column shows how far the body clock still trails (east) or runs ahead (west). On the Paris to New York trip, 6 hours ahead becomes 4.5, 3, 1.5, and on the fifth night the body clock has caught up.
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