Jet Lag Calculator

How the Jet Lag Calculator works

Jet lag happens because your internal clock, which sets when you feel sleepy, hungry and alert, keeps running on home time after you land. Daylight, meals and activity gradually pull it into line with local time, but only by a limited amount each day. Until it catches up you may be wide awake at 3 am, fading in the middle of the afternoon, and short on sleep for days.

How the jet lag calculator estimates recovery

The calculator finds the difference between the two time zones today, including daylight saving. A shift of more than 12 hours is treated as the shorter way round the clock, so a 14-hour eastward gap counts as 10 hours westward.

It then applies the rates most often used in circadian research and travel health guidance: the body clock adjusts by about one hour a day after flying east and about one and a half hours a day after flying west. Recovery in days is the shift divided by that rate, rounded up.

• Time shift shows the hours and direction.
• Recovery gives the estimated days to adjust.
• Your body thinks bedtime is shows when your usual bedtime falls in local time on arrival.
• Light gives the light advice for that direction.

Reading the day-by-day table

The table shows, for each day after arrival, the local times at which your body clock will feel like bedtime and waking, and how far it is still behind or ahead. Flying London to Tokyo in October is an 8-hour eastward shift. On day 1, a body used to 23:00 bedtime feels sleepy at 07:00 Tokyo time and wants to wake at 15:00. Each day that moves an hour earlier, until day 9, when you are adjusted. London to New York is a 5-hour westward shift: your body wants to sleep at 18:00 local time on day 1, and about four days later you are back to normal.

Why east is harder than west

Most people's natural body clock runs slightly longer than 24 hours, so staying up later comes more easily than going to bed earlier. Flying west lengthens your day, which works with that tendency. Flying east shortens it, asking your body to sleep before it is ready. That is why the same number of time zones usually takes longer to recover from heading east.

Very large shifts can confuse the body. After crossing nine or more zones, the clock sometimes shifts the long way round, so an eastward flight is treated by the body like a westward one. The calculator adds a note when this may apply.

Using light to adjust faster

Light is the strongest signal for resetting the body clock. Light in the morning moves your clock earlier, which helps after flying east. Light in the late afternoon and evening moves it later, which helps after flying west. Bright light at the wrong time can push the clock the wrong way, so:

• After flying east, get outside in the morning and avoid bright light in the late evening for the first few days.
• After flying west, get afternoon and evening light and keep early mornings dim.
• On very long eastward trips, avoid early morning light for the first day or two, when it falls in your body's evening, then seek it.

Other ways to reduce jet lag

• Shift your sleep by an hour a day for a few days before you travel, towards the destination's schedule.
• Eat meals on local time from the first day.
• Stay hydrated and go easy on alcohol, which fragments sleep.
• Use caffeine in the morning only, and avoid it within about six hours of local bedtime.
• Keep naps short, under 30 minutes, so they do not delay your night's sleep.
• For short trips of two or three days, consider staying on home time instead of adjusting.

Limits of the estimate

The rates are averages. Age, how much daylight you get, how well you slept on the flight and individual chronotype all change the real recovery time. The calculator uses today's offset between the zones, so if you travel across a daylight saving change the shift can differ by an hour. It is a planning guide, not medical advice. Anyone with a sleep disorder, or taking medication on a strict schedule, should ask a doctor before a long trip.

How many days jet lag lasts by destination

The formula is recovery days = time shift ÷ daily adjustment rate, rounded up. The body shifts about one hour a day flying east and about one and a half hours a day flying west. For example, London to Tokyo is +8 hours and takes about 8 days, while London to New York is −5 hours and takes about 4. Each example below uses the clocks in force today, so figures change by an hour when daylight saving starts or ends.

Jet lag flying from London, with a usual bedtime of 23:00

Related tools