What the Jet Lag Adjustment Planner does
This planner turns a flight into a day-by-day jet lag plan: it works out the time difference and which way your body clock has to move, then lists when to sleep, when to seek bright light and when to avoid it, for the days before you fly and the days after you land.
Every step follows a visible rule taken from published guidance - the CDC Yellow Book's advice to shift sleep by an hour a day before departure, and the rule of thumb from circadian research that the body clock adjusts by roughly an hour a day going east and an hour and a half going west. It is general guidance, not a medical or personalised model.
How to use it
- Choose your home time zone and the destination time zone. The time difference is taken on your arrival date, so daylight-saving changes at either end are included.
- Enter the local arrival time from your ticket, and the departure time too if you want the flight length shown.
- Enter your usual bedtime and wake time. They anchor the light windows, because the body clock's most sensitive point is estimated from when you normally wake.
- Pick how many days before the flight you can start shifting (none to three), and leave the direction on automatic unless you have a reason to go the long way round.
- Follow the table day by day, and copy, download or print it to take with you.
Reading the results
Advance means your body clock has to move earlier: you will want to sleep and wake earlier than usual, which is what flying east asks of you. Delay means moving later, which is what flying west asks. Delaying is usually easier, which is why westward trips adjust faster.
Seek light is the three-hour window after your estimated body-temperature minimum (for an advance) or before it (for a delay). Bright light, ideally daylight, in that window pushes the clock in the direction you need. Avoid light is the opposite window: light there pushes the clock the wrong way, so stay indoors or wear sunglasses if you are awake.
Hours left is how much shift remains at the end of each day. When it reaches zero the table switches to normal daylight - you are, on this rule, adjusted.
Worked example: London to Tokyo in October, starting two days early
A traveller leaves London at 11:30 on 9 October 2025 and lands in Tokyo at 09:00 on 10 October. London is on summer time (UTC+1) and Tokyo is UTC+9, so Tokyo is 8 hours ahead and the clock must advance by 8 hours. The flight takes 13 hours 30 minutes.
They usually sleep 23:00 to 07:00 and start two days early. On 8 October they sleep 22:00 to 06:00 and on 9 October 21:00 to 05:00, which takes 2 of the 8 hours before departure.
That leaves 6 hours to shift at 1 hour a day, so the plan runs six days after arrival. On arrival day the estimated temperature minimum is 04:00 home time minus the 2 hours already shifted, which is 02:00 in London or 10:00 in Tokyo. So the traveller avoids light from 07:00 to 10:00 and seeks it from 10:00 to 13:00. Each later day both windows move an hour earlier. By 16 October the minimum is back at 04:00, three hours before a 07:00 wake-up, and ordinary daylight is fine.
Formulas and scoring rules
- Time difference
diff = destination UTC offset - home UTC offset, at the arrival instantNormalised to more than -12 and up to +12 hours; positive means east.- Shift needed
shift = diff, or diff - 24 if delay is forced, or diff + 24 if advance is forced- Days after arrival
days = ceil((|shift| - pre-flight days x 1 h) / rate)rate = 1 h a day when advancing, 1.5 h a day when delaying.- Body-temperature minimum
CBTmin = usual wake time - 3 h - shift achieved so farConverted to destination time after arrival.- Light windows
advance: seek CBTmin to CBTmin + 3 h, avoid CBTmin - 3 h to CBTmin; delay: the reverse
Limitations: what the result does not prove
- Adjustment rates vary a lot from person to person. One hour a day east and one and a half west are averages from research on healthy adults, not a promise for you.
- The body-temperature minimum is estimated from your usual wake time, not measured. Late sleepers and early risers differ, and so do people after several nights of poor sleep.
- The plan does not cover melatonin, caffeine, sleeping pills or medical conditions. The CDC notes that melatonin timing matters and that product quality varies - ask a doctor or pharmacist first.
- For stays of two or three days, many people do better keeping to home time than adjusting at all. The plan assumes you want to adjust to the destination.
Privacy: where your data goes
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Standards and sources
- CDC Yellow Book - Jet lag - checked 19 Sep 2026
- Eastman and Burgess (2009) - How to travel the world without jet lag, Sleep Medicine Clinics
- IANA Time Zone Database
Frequently asked questions
Why is flying east harder on jet lag than flying west?
The human body clock naturally runs slightly longer than 24 hours, so it is easier to stay up later (delay) than to fall asleep earlier (advance). Flying east asks you to advance, which is why this planner assumes 1 hour a day east and 1.5 hours a day west.
What does the seek light window mean if it falls while I am asleep?
Do not set an alarm for it. Light only works when your eyes are open, so get bright light as soon as you wake if the window is still running. The windows move each day, and within a few days they fall into your waking hours.
Should I shift my sleep before I travel?
The CDC Yellow Book suggests moving bedtime by an hour a day for a few days before departure if you can. The planner lets you choose none to three days and shows how much of the shift that removes before you fly.
Why does the planner offer to delay when I am flying 9 or more hours east?
For very large eastward shifts, some people's clocks drift the long way round, delaying instead of advancing. If your schedule suits late nights, you can force a delay and see the plan; it will be longer but may feel easier.
Does the plan handle daylight-saving time changes?
Yes. The time difference is measured on your arrival date using the IANA time-zone rules in your browser, so a trip taken just before or after a clock change uses the offsets that actually apply then.
Is a jet lag plan worth it for a short trip?
For a shift of under three hours most people adjust within a day or two by keeping to local meal times and getting daylight, and the planner says so. For a stay of a couple of days across many zones, staying on home time can be better.
Last reviewed by the A2Z.Tools team against the sources listed above.
