Jet Lag Calculator

From New York to London your body clock has to move 5 hours earlier, and with 3 days of prep you match local time in about 2 days

Jet lag is the gap between your body clock and the clock on the wall at your destination. The US Centers for Disease Control and Prevention (CDC) puts the average pace at 1.5 hours a day after flying west and 1 hour a day after flying east, and says that shifting your sleep by 1 hour a day for the 2 to 3 days before you leave can cut the time you need to adjust. This calculator counts how many days that takes and lays out a day-by-day plan: when to sleep, when to seek bright light, and when to limit it.

Take a flight that leaves New York at 7:00 PM on October 19, 2026 and lasts 7 hours. You land in London at 7:00 AM on October 20. On that date London is 5 hours ahead of New York (the UK is still on British Summer Time until October 25, and New York is on daylight time until November 1), so your body clock has to move 5 hours earlier. If you start 3 days before departure and go to bed and get up 1 hour earlier each day, you cover 3 of those 5 hours before you board. The remaining 2 hours close at 1 hour a day, so you match London time in about 2 days. The whole 7-hour flight falls inside the London night (12:00 AM to 7:00 AM), so try to sleep for most of it.

Why a plan makes it easier

Jet lag happens when your body clock still follows the light and dark of home while your day has to run on local time. You may lie awake at night, feel sleepy in the afternoon, have an unsettled stomach or find it hard to concentrate. The CDC describes jet lag disorder as daytime impairment or physical discomfort within 1 to 2 days of travel.

• Light moves the clock: the same sunshine can move your body clock earlier or later, depending on when you get it. So "get more sun" is not enough advice. The times to seek light and to limit it change from day to day.
• The days before you leave shorten the days after you land: every hour you move in advance is an hour you do not have to close at your destination. Eastman and Burgess (2009) explain that even a day or two of shifting reduces the gap you face on arrival.
• On a short trip, it is better not to adjust: the CDC advises staying on home time for trips under 2 days, because you would barely start adapting before you head back and have to shift again.

How the calculation works

1. Time difference: the difference between the real UTC offsets of both places at the moment you land (destination minus home). Daylight saving is applied for the actual date using the IANA time zone data built into your browser.
2. Fold it to ±12 hours: clocks repeat every 24 hours, so a place 16 hours behind is the same as one 8 hours ahead. A positive folded value means the destination clock is ahead (an eastbound-type trip, where you move your body clock earlier); a negative value means it is behind (a westbound-type trip, where you move it later).
3. Direction: westbound-type trips are always "later". For eastbound-type trips, the calculator first applies your pre-trip prep. If the gap left on arrival is under 8 hours, it moves the clock earlier; if it is 8 hours or more, it moves the clock later by 24 hours minus that gap. This applies the Eastman and Burgess (2009) rule of thumb (shift earlier for up to 7 time zones east, later for 8 or more) to the gap remaining on arrival. You can also pick the direction yourself.
4. Prep before departure: for the number of days you choose (0 to 3), you move your bedtime and wake-up time 1 hour a day (CDC). The calculator never moves you further than the gap you need to close.
5. Days to adjust after arrival: the remaining gap divided by the daily pace (1 hour for earlier, 1.5 hours for later), rounded up. It assumes you keep your usual bedtime and wake-up time on local time from the first day.
6. Light windows: your body-clock low point (the minimum of core body temperature) is estimated at 3 hours before your usual wake-up time. For each day, the calculator moves it by the gap still to close and takes the 7 hours on either side, keeping only the hours you are awake. Light after the low point moves the clock earlier and light before it moves the clock later, so those windows become "seek bright light" or "limit light" depending on your direction.

Pace and prep follow the CDC Yellow Book 2026 (checked October 12, 2026). The direction rule and the low-point estimate follow Eastman and Burgess (2009).

Days to adjust by time difference (departing from New York)

This assumes you usually sleep from 11:00 PM to 7:00 AM. "3 days of prep" means moving your sleep 1 hour a day for up to 3 days before you leave, only as far as needed. "Mar–Nov" means the months New York is on daylight time (roughly mid-March to early November) and "Nov–Mar" means standard time. For a few weeks in March and in late October, US and European clocks change on different dates, so the gap is an hour different.

The +8 hour row changes direction with prep. Arriving with no prep would mean moving the clock 8 hours earlier, and that is where the body is more likely to settle the other way around, so the calculator plans 16 hours later instead. But if you move 3 hours earlier before you leave, only 5 hours are left on arrival, and moving earlier becomes the faster route. That is why prep matters most on long eastbound trips such as New York to the Gulf or India.

When to get light and when to avoid it

The CDC explains that light in the morning after your body-clock low point (usually 2 to 4 hours before your normal wake-up time) moves the clock earlier, and light in the evening, before the low point, usually moves it later. Eastman and Burgess (2009) say the same: light after the low point advances the clock and light before it delays it. The catch is that right after you land, your body-clock low point sits at the wrong time on the local clock.

• Eastbound-type, New York to London on October 19 (5 hours earlier, 3 days of prep): on your first morning your low point is around 6:00 AM London time, so the light after you wake at 7:00 AM moves your clock earlier. Get outside from 7:00 AM until about 1:00 PM. On day 2 the low point slides to 5:00 AM, so you also trim light between 10:00 and 11:00 PM, just before bed. By your third night you are adjusted.
• Westbound-type, New York to Honolulu on October 19 (6 hours later, 3 days of prep): your low point on the first night is around 1:00 AM Honolulu time, so light in the evening (6:00 to 11:00 PM) moves your clock later. Wear sunglasses from 7:00 to 8:00 AM, because light just after the low point would pull it back earlier. On day 2 the low point moves to 2:30 AM: seek light from 7:30 to 11:00 PM and limit it from 7:00 to 9:30 AM. By the third night you are adjusted.

"Limit light" in the plan does not mean staying in the dark. As the CDC suggests, drawing the curtains or wearing dark sunglasses is enough.

On the plane and on your first day

• Sleeping on the plane: the UK National Health Service (NHS) advises sleeping on the flight if it is night at your destination. The calculator shows the part of the flight that overlaps your usual sleep time at the destination as "sleep on the plane".
• Alcohol and caffeine: the CDC notes that alcohol may help you fall asleep faster but wakes you more often, and that both alcohol and caffeine can dehydrate you. Drink plenty of water.
• Naps: if you are sleepy on your first day, nap for only 20 to 30 minutes. A long nap makes it harder to fall asleep that night (CDC).
• Last caffeine: the CDC says to stop caffeine 6 hours before bed. If you go to bed at 11:00 PM, that is 5:00 PM.

Short trips and the way home

If you stay one night, the calculator keeps you on home time, because the CDC advises not adapting on trips under 2 days. For stays of 2 nights or more it works out how far you get before you leave, and if you leave before you are fully adjusted, it tells you how many hours are left. Those hours are the ones you will not have to close again at home. To plan the trip back, press "Swap origin and destination" and run the same calculation.

Melatonin

The CDC explains that melatonin taken when your body clock thinks it is early evening moves the clock earlier, and melatonin taken when it thinks it is morning moves it later. It says 0.5 to 1 mg is often enough, does not recommend high doses above 5 mg, and warns that taking it at the wrong time can leave you more out of step. In the United States, melatonin is sold as a dietary supplement and is regulated less strictly by the Food and Drug Administration (FDA) than drugs. The National Center for Complementary and Integrative Health (NCCIH) notes that some products do not contain what the label says, that reviews suggest melatonin may help with jet lag, and that short-term use appears safe for most people while long-term safety information is lacking. It also says people who take blood thinners or have epilepsy need medical supervision, and that pregnancy and breastfeeding have not been well studied. This calculator does not build a melatonin schedule. If you want to try it, talk to a doctor or pharmacist first.

Common mistakes

• Choosing the direction from the sign alone: in summer Tokyo is 13 hours ahead of New York, but going around the clock that is the same as 11 hours behind, so it is a "later" trip, not a 13-hour jump east.
• Seeking early morning sun on an eastbound trip, no matter what: if your low point is still in the morning, light before it pushes your clock the wrong way. Wait until after the low point.
• Sleeping for hours on your first afternoon: it pushes that night's sleep later and slows adjustment. Cut naps to 20 to 30 minutes.
• Staying up all night before the flight to sleep on the plane: missing sleep makes jet lag worse (CDC). Prep means moving your sleep times, not cutting your sleep.
• Adjusting to local time on a one-night trip: you would be getting used to it just as you head home, and you would be out of step twice.

Limits and cautions

• The pace of 1 hour and 1.5 hours a day is an average across studies. Whether you are a morning or evening person, your age and how much light you actually get can make you a day or two faster or slower.
• The 1.5-hours-a-day pace for moving later is the CDC average after westward flights; the calculator uses the same pace when an eastward trip is matched by moving later. A daylight saving change during your stay is not added to the remaining gap.
• The body-clock low point is estimated at 3 hours before your usual wake-up time (the CDC gives 2 to 4 hours; Eastman and Burgess give 3 to 4.5). The 7-hour light window is also an estimate, chosen to cover the 3.5 hours after waking in the shift-earlier studies and the 2 hours before bedtime in the shift-later studies.
• The calculator does not model how your clock moves during the flight or during a long layover. For a long connection, add the layover to the flight time.
• Shift workers, people with a sleep disorder and people who take medication may find this schedule does not fit. It does not replace diagnosis or treatment.
• Local times come from the time zone data in your browser. Right after a government changes its daylight saving rules, the data can be off by an hour until it is updated. For example, British Columbia (Vancouver) is keeping daylight time all year from 2026, so it stays on UTC−7 (IANA time zone data 2026b), but a browser with older data would calculate UTC−8 from November. That is why this calculator fixes Vancouver at UTC−7.

Sources

• 01CDC Yellow Book 2026: Jet Lag DisorderAdaptation of 1.5 hours a day westward and 1 hour a day eastward, shifting sleep 1 hour a day 2 to 3 days before departure, home time for trips under 2 days, morning and evening light, naps, caffeine, melatonin 0.5 to 1 mg
• 02Eastman CI, Burgess HJ. How to travel the world without jet lag. Sleep Med Clin 2009Body-clock low point 3 to 4.5 hours before waking, effects of light before and after the low point, shifting earlier up to 7 time zones east and later for 8 or more
• 03NHS: Jet lagSleeping on the plane if it is night at the destination, drinking water, cutting down caffeine and alcohol, getting outside by day
• 04NCCIH (NIH): Melatonin, What You Need To KnowIn the US melatonin is a dietary supplement regulated less strictly than drugs, label accuracy, evidence on jet lag, short-term safety
• 05IANA: Time Zone DatabaseCity UTC offsets and daylight saving rules (the source of the browser's Intl time zone data); 2026b: British Columbia on UTC−7 all year

Frequently asked questions

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