How to use
- 1Choose the departure and arrival time zones, or type airport codes such as JFK or LHR.
- 2Enter the local departure time and the flight duration.
- 3Set your usual bedtime and wake-up time.
- 4Follow the plan: shifted sleep times before you leave, then when to seek and avoid bright light after arrival.
Why the direction of travel matters
Your body clock runs on a cycle of roughly 24 hours that daylight resets gradually. Flying west lengthens your day, which the body clock follows fairly easily; flying east shortens it, which is usually harder. The calculator uses commonly cited adjustment rates of about 1.5 hours per day after westward flights and 1 hour per day after eastward flights, and shows the estimated number of days you will need. Below 3 hours of difference, symptoms are usually mild and the plan is optional. For eastward trips of more than 9 hours, some people adapt faster by delaying the body clock instead, as if flying west.
How the plan is built
Before departure, the plan shifts your sleep by one hour per day for up to three days — earlier before an eastward trip, later before a westward one — so part of the adjustment happens at home. After arrival it estimates where your body clock is each day and times light around your body-temperature minimum, a few hours before your usual wake time, when light has the strongest effect: light after that point moves the clock earlier (helping eastward), light before it moves the clock later (helping westward). Light windows are limited to your waking hours, and the arrival time is calculated with real time-zone rules, including daylight saving time.
Practical tips
On the plane, set your watch to destination time and sleep if it is night there. After arrival, eat and sleep on local time, keep naps to 20–30 minutes, drink caffeine only in the morning, limit alcohol and stay hydrated. Some travellers use melatonin; whether it suits you, and the dose and timing, is a question for your doctor. The plan is general information based on the CDC Yellow Book jet lag chapter and Eastman & Burgess (2009), not medical advice.
Frequently asked questions
How long does jet lag last?
It depends on how many time zones you cross and in which direction. A common rule of thumb is about one day per time zone, and eastward trips usually take longer to recover from than westward ones.
Is jet lag worse flying east or west?
Usually east. For most people the body clock naturally runs slightly longer than 24 hours, so delaying it (westward) is easier than advancing it (eastward).
Can I prevent jet lag completely?
Rarely, but you can shorten it by shifting your sleep before departure and timing light exposure after arrival, as the plan suggests.
Does the calculator handle daylight saving time?
Yes. The time difference is calculated for your actual travel date using the IANA time zone database, so DST changes are taken into account.
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