Free Online Jet Lag Calculator

How to use

1. 1

   Enter your trip. Type where you are flying from and to: a city, an airport code such as JFK, or a time zone such as Europe/Paris. From starts on your current time zone.

2. 2

   Choose your arrival date. Pick the date you land, in local time there. Daylight saving time is worked out for that date.

3. 3

   Add your usual sleep and a head start. Open Your usual sleep to set your normal bedtime and wake time, and choose whether to start adjusting 1, 2 or 3 days before you fly.

4. 4

   Follow the plan. Each day shows when to sleep, when to seek light and when to avoid it, in that day’s local time, up to the day your body clock should be adjusted.

Why jet lag happens

Your body keeps its own clock, which runs sleepiness, alertness, body temperature and appetite on a cycle of about 24 hours. After a fast trip across several time zones, that clock is still on home time while the sun and the local clocks are not, and the mismatch is jet lag: poor sleep at night, sleepiness in the day, a foggy head and often an unsettled stomach. It lasts until the internal clock has moved to local time, and light is the strongest signal it uses to move.

Which way light pushes the clock depends on when it arrives. Every body clock has a low point in the early hours, when body temperature bottoms out and sleepiness peaks, usually a few hours before you normally wake. Bright light after that low point moves the clock earlier; bright light in the hours before it moves the clock later. Flying east, you need to move earlier, and flying west, later. Because most people’s internal day runs slightly longer than 24 hours, moving later comes more easily, which is why flying west is usually the easier direction.

How fast the body clock can move

Left to itself, the body clock shifts about an hour a day after flying east and about an hour and a half a day after flying west. Those are the average rates in the CDC Yellow Book, and the rates this calculator plans with. A six-hour shift therefore takes about six days eastward and four westward. Starting before you fly shortens the wait: the CDC suggests moving your sleep an hour earlier each day before an eastward trip, or an hour later before a westward one, for two or three days beforehand.

Very long trips east work differently. Moving a body clock 11 hours earlier at an hour a day takes 11 days, while moving it 13 hours later at an hour and a half a day takes about 9, and bodies often go that way on their own: Helen Burgess notes that after eastward flights across eight or more time zones, the clock often shifts later instead. Researchers call this antidromic re-entrainment. The calculator plans it on purpose for eastward shifts of 10 hours or more, which is why a trip from London to Sydney in January shifts your clock later even though Sydney is 11 hours ahead.

Light is the lever you control

Seeking light means getting bright light into your eyes. Daylight outdoors without sunglasses is best; a bright window or a light box helps indoors. Avoiding light means the opposite: staying inside away from windows, keeping lamps low, and wearing dark sunglasses if you have to go out. Light at the wrong time can pull the clock the wrong way, which is why the plan gives you both windows.

Flight times matter for the same reason. The CDC Yellow Book suggests choosing a flight that lands when you can get light at a helpful time, and timing meals, sleep and light to the destination from the moment you board. After you land, naps of 20 to 30 minutes can keep you going without spoiling the night; long daytime naps make the first nights harder.

How this calculator works

The calculator starts from the time difference between the two places on your arrival date, so daylight saving time is handled for that date. If the difference is an hour or less, it does not make a plan. Otherwise it plans at 1 hour a day for shifting earlier and 1.5 hours a day for shifting later, and picks whichever direction finishes sooner: westward trips always shift later, eastward trips of up to 9 hours shift earlier, and eastward trips of 10 hours or more shift later, the long way round.

It estimates your body clock’s low point as 2 hours before your usual wake time and moves it by each day’s progress. To shift earlier, each day seeks light for the 3 hours after the low point and avoids it for the 3 hours before; to shift later, it is the reverse. Both windows are trimmed to the hours you are awake. If less than an hour of seek-light time is left, the plan falls back to the first 2 hours after waking when shifting earlier, or the last 2 hours before bed when shifting later, and an avoid-light window shorter than 30 minutes is dropped, since you would be asleep for most of it. From the first day after landing, sleep is your usual bedtime and wake time in local time. Each day you start early at home moves your sleep another hour toward the destination and counts as an hour of the shift. The plan ends on the first day the whole shift is done, or after 14 days.

These are estimates for healthy adults, not medical advice. Body clocks differ, and the low point in particular can fall anywhere from about 2 to 4 hours before waking, so treat the windows as a guide rather than a timetable. If you have a sleep disorder, are pregnant, or take medicines that affect sleep, talk to a clinician before a long trip, especially about melatonin or sleeping pills.

Link parameters

Every input travels in the address bar, so a link can open the calculator with a trip already planned.

Frequently Asked Questions