Time & Sun
World Jet Lag Calculator
Estimate the time-zone shift between two cities and the days you will likely need to recover, with east- versus westbound guidance.
London → Sydney
Time difference
+10 h
Direction
Eastbound
Est. recovery
10 days
Travelling east is harder on the body clock — plan to arrive a day early if you can. Allow roughly one day per hour of offset to fully adapt; pre-shifting sleep by an hour a day for several days before departure helps.
Estimate only — individual recovery varies with age, chronotype, sleep and light exposure.
What jet lag actually is
Jet lag is the mismatch between your internal circadian clock and the local clock at your destination. Your body runs on a roughly 24-hour rhythm set by light exposure, meal timing and sleep. Crossing time zones faster than the clock can adjust leaves you alert when locals sleep and exhausted when they are active.
The symptoms go beyond tiredness: poor concentration, digestive upset, low mood and reduced physical performance. For athletes and executives, jet lag can measurably impair output for several days after a long flight. Understanding the size of the shift helps you plan recovery.
The one-day-per-hour rule of thumb
A common rule is that the body adjusts by about one hour of offset per day. A six-hour shift therefore takes roughly six days to fully resolve. This tool uses that rule to produce its estimate. It is an average — some people adapt faster, others slower — but it is a reliable planning guide for most travellers.
The estimate reflects the offset in hours between your origin and destination right now. Because DST can change the offset by an hour on certain dates, the figure is computed from live IANA offsets rather than a fixed longitude difference.
Why eastbound is harder than westbound
The natural human circadian period is slightly longer than 24 hours — about 24 hours and 12 minutes for most people. This means it is easier to delay sleep (go to bed later, as on a westbound flight) than to advance it (wake earlier, as eastbound). Eastbound travellers typically recover more slowly for the same number of hours of shift.
A westbound trip of six hours might take four or five days to settle, while an eastbound trip of the same six hours might take six or seven. Planning extra recovery time after eastbound flights — or arriving a day early — is prudent for important commitments.
Pre-shifting sleep before departure
The most effective jet-lag strategy is to begin adjusting before you fly. Shift your bedtime and wake time by about an hour per day for several days before departure — earlier for eastbound travel, later for westbound. Combined with timed light exposure (bright light in the morning for eastbound, evening for westbound), this can cut recovery time substantially.
On the aircraft, set your watch to destination time immediately and begin eating and sleeping on the destination schedule. Avoid heavy meals and alcohol, which disrupt the sleep you need most on arrival. Hydration matters more than usual because cabin air is very dry.
Light, melatonin and meals
Light is the strongest cue for the circadian clock. After eastbound travel, seek bright morning light to advance the clock; after westbound travel, seek evening light to delay it. Avoiding light at the wrong time (blue light in the evening when you need to advance, for example) is just as important.
Timed melatonin (a low dose, taken at the destination bedtime for several nights) can help reset the clock and is supported by evidence for eastbound travel. Regular meal times on the destination schedule also act as a cue. Together, light, melatonin and meals can shorten recovery by a day or two.
Planning around jet lag
For a short trip (two or three days), full adaptation may not be worth pursuing — you will leave before you adjust. In that case, keep your origin schedule if it suits your meetings, or use caffeine and short naps to bridge the gap. For longer stays, invest in full adaptation using the strategies above.
If you have a critical meeting or performance, arrive a day or two early if possible. The single most reliable jet-lag mitigation is simply giving your body time. This calculator tells you how much time that is likely to be, so you can decide whether to build it into your itinerary.
Frequently Asked Questions
How is the recovery estimate calculated?
We take the current UTC offset of each city from the IANA database, compute the difference in hours, and assume roughly one recovery day per hour of shift. This is a widely used rule of thumb, not a precise prediction.
Why is eastbound travel harder?
The human circadian clock runs slightly longer than 24 hours, so it is easier to delay sleep (westbound) than to advance it (eastbound). Eastbound travel against the clock typically takes longer to recover from for the same number of hours.
Can I reduce jet lag before I fly?
Yes. Shift your sleep by about an hour per day for several days before departure — earlier for eastbound, later for westbound. Morning light advances the clock; evening light delays it. Hydration and avoiding alcohol on the flight also help.
Does the calculator account for daylight saving?
Offsets are read live from the IANA database for today, so any current DST is included. The shift shown is the actual offset difference at the moment you check.
Is one day per hour accurate for everyone?
No. It is an average. Older adults, evening chronotypes and those who sleep poorly recover more slowly; morning types and younger people often adapt faster. Use the estimate as a planning guide, not a guarantee.
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