Jet Lag Calculator
Enter your flight and your usual sleep times. You get a day by day plan: how to shift your sleep before you fly, when to sleep on the plane, and when to get or avoid bright light after you land.
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You land in London at 7 in the morning and your body is certain it is 2 AM. The meeting is at 10. The hotel room is not ready until 3.
Jet lag is your body clock still running on home time. It does catch up by itself, but slowly: about an hour a day after flying east and an hour and a half after flying west, according to the CDC's guide for travel doctors. What you do with sleep and light in the days around the flight decides how long it drags on.
Enter your flight and your usual hours. The calculator builds a plan for each day: when to go to bed, when to get bright light, when to keep it dim, and roughly when you should feel normal again. Daylight saving and the date line are handled for you.
How the plan is worked out
The plan rests on four findings from the CDC Yellow Book's chapter on jet lag and the research behind it.
- Your body clock has a daily low point. It usually falls 2 to 4 hours before your normal wake time. The calculator puts it 3 hours before, the estimate used by sleep researchers Charmane Eastman and Helen Burgess.
- Light on either side of it pushes the clock opposite ways. Bright light after the low point, usually the morning, moves your clock earlier. Light before it, usually the evening, moves it later. The same morning sunshine can help or hurt, depending on where your body clock is.
- The clock moves about an hour a day flying east and an hour and a half flying west. East is harder because the body's natural day runs a little longer than 24 hours, so staying up later comes easier than going to bed earlier.
- Starting early helps. Moving your sleep an hour a day toward the new time zone for 2 or 3 days before the trip cuts the time it takes to adjust.
Why the plan sometimes says to avoid morning light
After a big jump east, local morning can arrive before your body's low point. Bright light then pushes your clock the wrong way, later instead of earlier, and Eastman and Burgess note that this backwards shift is common after flying 8 or more time zones east. So for the first days after a flight like Los Angeles to London, the plan may tell you to keep light dim early in the morning and get bright light from late morning, with the window moving earlier each day as your clock catches up.
When the long way round is quicker
New York to Tokyo reads as 13 hours ahead on the clock, but for your body that lands in the same place as 11 hours behind. Shifting 11 hours later at an hour and a half a day beats 13 hours earlier at an hour a day, so the plan goes that way and tells you so.
Short trips
If you are only staying a day or two, the CDC's advice is not to adapt at all. Keep your sleep, meals and work on home time as much as you can. Enter a stay of under 2 days and the calculator switches to that advice.
How many days does jet lag last?
Rough days to adjust with no preparation, at the CDC's average rates of 1 hour a day flying east and 1.5 hours a day flying west.
| Time difference | Flying east | Flying west |
|---|---|---|
| 2 hours | About 2 days | About 2 days |
| 3 hours | About 3 days | About 2 days |
| 4 hours | About 4 days | About 3 days |
| 5 hours | About 5 days | About 4 days |
| 6 hours | About 6 days | About 4 days |
| 7 hours | About 7 days | About 5 days |
| 8 hours | About 8 days | About 6 days |
| 9 hours | About 9 days | About 6 days |
| 10 hours | About 10 days | About 7 days |
| 11 hours | About 9 days | About 8 days |
| 12 hours | About 8 days | About 8 days |
At 11 or 12 hours east, shifting later the long way round is quicker, which is why those rows come back down. Two or three days of shifting your sleep before you fly take days off both columns. Everyone adjusts at a slightly different pace, so treat these as averages.
Melatonin and sleep medicine
The CDC's guidance for travel doctors covers both, and both depend on getting the details right. Melatonin can help shift the body clock, but taken at the wrong time it can make the mismatch worse, and there is limited data on how it interacts with other medications. This calculator does not give dosing advice. If either interests you, ask a doctor or pharmacist before your trip.
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Questions
How long does jet lag last?
Roughly a day for every time zone you cross flying east, and a little less flying west. The CDC's guide for travel doctors puts the body clock's natural adjustment at about 1 hour a day eastward and 1.5 hours a day westward, so 6 hours east takes around 6 days and 6 hours west around 4. Shifting your sleep before the trip shortens both.
Is jet lag worse flying east or west?
East, for most people. Flying east asks your body to go to bed earlier, and the body clock naturally runs a little longer than 24 hours, so moving later comes easier than moving earlier. That is why the body adjusts about 1 hour a day after flying east but 1.5 hours a day after flying west.
How do I prepare for jet lag before a flight?
Start 2 or 3 days before you fly. Move your bedtime and wake time 1 hour a day toward the destination: earlier if you are flying east, later if you are flying west. Get bright light in the morning when shifting earlier and in the evening when shifting later. The calculator lays out the exact times for your trip.
Should I adjust to the new time zone on a short trip?
Not if you are staying less than about 2 days. The CDC advises keeping your sleep, meals and activity on home time for short trips, since your body would barely start adjusting before you fly back.
Should I sleep on the plane?
Sleep if it is night at your destination, and try to stay awake if it is daytime there. If you need a nap, keep it to 20 or 30 minutes, since the CDC notes that longer daytime naps can get in the way of sleep that night. The calculator shows which part of your flight falls in the destination's night.
Does melatonin help with jet lag?
It can, but timing matters. The CDC's guidance for travel doctors notes that melatonin taken at the wrong time can make the mismatch worse, and that there is limited data on how it interacts with other medications. Ask a doctor or pharmacist before using it.
This calculator gives general information built on published averages. It is not medical advice.