Jet Lag Calculator: Your Personal Sleep and Light Plan

Tell the calculator how far ahead or behind your destination is and when you usually sleep, and it gives you a day-by-day plan for the trip: when to get bright light, when to avoid it, and roughly how many days your body clock needs to catch up. Fly six hours east, for example, and the average rate of about an hour a day means around six days ^[1]. The plan also tells you when a trip is too short to bother adapting at all.

How the jet lag calculator works

Jet lag is your body clock still running on home time while every clock around you says something else. Light is the main tool for moving it, and timing is everything. Your clock has a turning point near the low point of your core body temperature, in the early hours of the morning. Light in the hours before that low shifts the clock later; light after it shifts the clock earlier ^[3]. Get the timing backwards and you push yourself the wrong way.

The calculator works through five steps. Some rest directly on a source, and some are SleepGuru's own simplifications of the research, flagged as such:

1. Direction. Flying up to 7 hours east, it plans to move your clock earlier. From 8 hours east, it plans to move it later, the long way round, because clocks often drift that way anyway after big eastward trips. Sleep researchers describe this 7-or-8 split as a general guideline that won't suit every traveler ^[2]. Flying west, it moves your clock later.
2. Days to adapt. It divides the shift by about 1 hour a day for earlier moves and 1.5 hours a day for later moves, the average rates in the CDC Yellow Book ^[1]. Assuming your clock moves by exactly that amount every day is our simplification.
3. Your body-temperature low. With an 8-hour sleep, this low usually falls about 3 to 4.5 hours before you wake ^[2]. The calculator picks 3.5 hours before your usual wake time, our own choice within that range, and converts it to local time.
4. Light windows. Each day it gives a 3-hour "seek light" window on the side of the low that moves you the right way, and a 3-hour "avoid light" window on the other side. The 3-hour width is our simplification, not a rule from the research.
5. Short trips. If you are staying less than 2 days, it tells you to stay on home time, as the CDC advises for short trips ^[1].

The suggested bedtime is simply your usual home bedtime on the local clock. That is our default, so move it if your trip demands something else.

Worked example: six hours east

Say you usually sleep 11:00 PM to 7:00 AM and fly to a city 6 hours ahead, such as New York to Paris for most of the year. Your estimated body-temperature low is 3:30 AM at home, which is 9:30 AM in Paris when you land. Your clock needs to move 6 hours earlier, at about an hour a day:

What does day 1 look like in practice? Stay out of bright morning sun until about 9:30, then get outside. By day 4 the morning window opens before you are up, so you just get light once you wake. Your target bedtime throughout is 11:00 PM local time.

Flying the other way, six hours west, the low lands at 9:30 PM local time on day 1. The plan has you seek light from 6:30 to 9:30 PM and avoid it from 9:30 PM to 12:30 AM, and on the average rate you should be adjusted in about four days.

Shifting your sleep before you fly

Choose the pre-flight option and the calculator moves your bedtime and wake time by 1 hour a day for up to 3 days before departure, in line with the CDC's suggestion of about an hour a day for 2 to 3 days ^[1]. Does it work? In a study of this approach before eastward travel, people who advanced their sleep 1 hour a day for 3 days with bright light in the first hours after waking moved their clocks about 2 hours earlier. With ordinary dim indoor light, the shift was only a little over half an hour ^[4]. So if you do it, pair it with morning light. For the six-hours-east example, three pre-flight nights at 10:00, 9:00 and 8:00 PM (waking at 6:00, 5:00 and 4:00 AM) cut the plan after landing to about three days. Counting the full hour a day as done is our simplification; even with continuous bright light, the study's average came to about 40 minutes a day.

Why east and west feel different

The average human body clock runs slightly longer than a day, about 24.2 hours ^[5]. Stretching a day is easier than shortening it, so most people re-adjust more slowly after moving their clock earlier than after moving it later ^[4]. That's why eastward trips get more days in the plan. The full story, including why some long eastward trips go the long way round, is in is jet lag worse flying east or west.

Sleep on the flight

The CDC suggests timing meals, sleep and light on the plane by the destination clock ^[1]. In plain terms: sleep if it is night where you are going, and try to stay awake if it isn't. Losing sleep while traveling can make jet lag worse, and keeping alcohol and caffeine low helps you sleep in flight ^[1]. An eye mask and earplugs can make an in-flight nap easier ^[1].

Limits and assumptions

The body-temperature low is an estimate from your usual wake time, not a measurement, so very early or very late sleepers may be off by an hour or more. The circadian rhythm calculator maps your own clock in more detail. The adaptation rates are averages too. Real people adapt faster or slower, and the rates assume you actually follow the light plan. Daylight outdoors is the easiest source, and our guide to how long to get morning sunlight covers how much is enough.

Safety comes before any light window. Jet lag can make you sleepy at odd hours, so if you feel drowsy after landing, don't drive, and never wear dark glasses at the wheel to "avoid light". The plan leaves out medication and melatonin on purpose; if you are thinking about either, talk to a doctor or travel health clinic. It is not meant for shift workers or anyone with a circadian rhythm disorder, and if jet-lag-like symptoms carry on well past the expected adaptation time, see a doctor.