About the Jet Lag Calculator
This jet lag calculator estimates how long your body clock will take to catch up with a new time zone and builds a simple day-by-day plan to speed it up. Choose where you are flying from and to and the travel date, enter your usual bedtime and wake time, and it works out the time difference (daylight saving time included), whether you need to advance or delay your body clock, and roughly how many days adjustment will take.
The plan tells you when to seek bright light and when to avoid it, which is the strongest signal for resetting your internal clock, and when your body will feel ready to sleep on arrival. You can also shift your bedtime by an hour a day for a few days before you leave, which reduces the jet lag you arrive with.
The estimates use widely quoted rules of thumb — the body clock shifts about one hour per day after flying east and about an hour and a half per day after flying west — so individual experience will vary. For large shifts of more than 12 hours the calculator plans the shorter adjustment in the other direction. This is general information, not medical advice.
With the default inputs, the days to adjust after arrival is 6 days. Change any value above to recalculate instantly.
How to use the jet lag calculator
- 1Choose the time zones you are flying from and to.
- 2Enter the travel date so daylight saving time is applied correctly.
- 3Enter your usual bedtime and wake time at home.
- 4Optionally choose a few days to shift your schedule before you leave.
- 5Follow the plan: sleep on local time, seek and avoid bright light as shown.
Formula and method
The time difference is taken from the UTC offsets of both places on your travel date. Flying east means your body clock must advance (go earlier); flying west means it must delay. If the difference is more than 12 hours, adjusting the other way round is shorter, so the shift is wrapped into the range −12 to +12 hours.
The body clock is assumed to adapt about 1 hour per day after eastward travel and about 1.5 hours per day after westward travel, and each day of pre-trip adjustment removes 1 hour. Light advice is timed around your core body temperature minimum, estimated at about 3 hours before your body-clock wake time: light in the 3 hours after it advances the clock, and light in the 3 hours before it delays the clock.
- Shift
- Hours the body clock must move (+ advance, − delay)
- Rate
- Hours of adjustment per day
- Tmin
- Core body temperature minimum, ≈ 3 h before natural wake time
Worked examples
New York to Paris (6 hours east)
Paris is 6 hours ahead of New York in mid-October, so your body clock must advance 6 hours. At about 1 hour per day that takes roughly 6 days, and on the first night your body will not feel sleepy until about 5:00 AM local time.
Same trip with two days of pre-adjustment
Going to bed an hour earlier on each of the two nights before departure moves your bedtime to 9:00 PM and removes 2 hours of the shift. The remaining 4 hours take about 4 days to adjust after landing.
Los Angeles to Tokyo (16 hours ahead)
Tokyo is 16 hours ahead of LA, but delaying the body clock by 8 hours is the shorter route. At about 1.5 hours per day that takes around 6 days, and at first you will feel sleepy in the mid-afternoon.
London to New York (5 hours west)
New York is 5 hours behind London, so the body clock needs to delay. Westward adjustment is easier — about 1.5 hours a day — so expect around 4 days, with early evening sleepiness at first.
Frequently asked questions
How long does jet lag last?+
A common rule of thumb is about one day per time zone crossed when flying east, and a bit less when flying west, where the body adjusts about 1.5 hours per day. Shifts under three hours usually cause only mild symptoms.
Is jet lag worse flying east or west?+
For most people flying east is harder, because it requires going to sleep and waking earlier than the body wants. The natural body clock runs slightly longer than 24 hours, which makes delaying (westward travel) easier.
How does light help with jet lag?+
Bright light is the main signal that resets the body clock. Light in the hours after your body temperature minimum shifts the clock earlier, while light before it shifts the clock later — so timing matters as much as the amount of light.
Should I adjust my sleep before I travel?+
Moving your bedtime and wake time about an hour a day toward the destination schedule for a few days before departure reduces the shift you arrive with. It works best for eastward trips crossing several time zones.
Does melatonin help with jet lag?+
Many studies suggest melatonin taken close to the target local bedtime can reduce jet lag symptoms, especially after eastward flights. Talk to a doctor or pharmacist about dose and timing, particularly if you take other medications.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.