Jet Lag: Shifting Your Clock Before You Fly
By Mark Fulton · 2026-09-17 · 10 min read
To adjust to jet lag before you travel, move your sleep toward your destination's clock by about one hour a day over your last few nights at home: earlier to bed and earlier up for an eastbound trip, later to bed and later up for a westbound one. Pair each step with light at the right end of the day, morning light when shifting earlier and evening light when shifting later, because light is the signal your body clock sets itself by. Four nights of one-hour steps closes four hours of the gap before you board. The plans below give the bed time, the alarm and the light for each night. This is scheduling, not medical advice: if you have a medical condition or take any medication, ask a clinician before you change your sleep.
Why does eastbound travel hurt more?
Flying east asks your body clock to run earlier, a phase advance. Flying west asks it to run later, a phase delay. The two are not symmetrical.
For most people the internal clock runs slightly longer than 24 hours, so drifting later comes naturally and being pulled earlier goes against the grain. The CDC Yellow Book chapter on jet lag disorder gives average adaptation rates of about 1.5 hours a day after westward travel and about 1 hour a day after eastward travel, and gives that longer rhythm as the reason.
Paris is six hours ahead of New York for most of the year. Midnight in Paris is 18:00 to a body still on New York time, so your first night there means trying to sleep in what your clock considers early evening.
Very long eastbound trips add a wrinkle. Charmane Eastman and Helen Burgess, in their review How to Travel the World Without Jet Lag, note that after crossing eight or more zones east the body clock often shifts the long way round, delaying instead of advancing, and badly timed light after arrival is one way that happens. For a trip that large, treat the plan below as a starting point.
How many days before should you start shifting?
Two to four. The Yellow Book's example shifts sleep one hour per day in the 2 to 3 days before the trip, which it says may reduce the time needed to adjust. The worked eastbound example in the Eastman and Burgess review starts four days out. The plans here use four nights. If your week will not allow that, start at Night 2 or Night 3. A partial shift still leaves less to close on arrival.
Protect total sleep: Cleveland Clinic lists poor sleep before a flight as something that raises your chances of jet lag, so the plans move bed time and wake time together and keep the night the same length. Going to bed early and still getting up at your usual time is not a shift. It is a short night.
And a short trip may not be worth it. The NHS suggests that for a 2 to 3 day trip you could keep your home eating and sleeping times rather than shift twice.
How much can you shift per day?
One hour. That is the step the Yellow Book uses, and it is the ceiling Eastman and Burgess recommend for shifting earlier. In their laboratory work, advancing the sleep schedule two hours a day produced only slightly more clock shift than one hour a day, and the difference was not statistically significant. For night owls they suggest half an hour.
Shifting later can go faster: the same review describes a study with bright light before bed where the clock delayed about two hours a day. The plans still use one hour in both directions, which keeps the westbound plan conservative and inside the Yellow Book's example. Each table carries a half-hour column for the gentler version.
What does a four-day plan look like?
Both plans use the same stated baseline: usually in bed at 23:00 and up at 07:00. Each row is one night, and the alarm is the next morning's. If your baseline differs, move your own times by the same steps.
Eastbound: New York to Paris, six hours ahead
New York time. One hour earlier per night.
| Night | Lights out | Alarm next morning | Half-hour version (bed / alarm) | Light the next day |
|---|---|---|---|---|
| Night 1 (four nights out) | 22:00 | 06:00 | 22:30 / 06:30 | Bright light after waking, outdoors if you can. Dim evening. |
| Night 2 | 21:00 | 05:00 | 22:00 / 06:00 | Same. |
| Night 3 | 20:00 | 04:00 | 21:30 / 05:30 | Waking before sunrise: bright indoor light. |
| Night 4 (night before the flight) | 19:00 | 03:00 | 21:00 / 05:00 | Morning light on flight day. |
A 19:00 to 03:00 night in New York is 01:00 to 09:00 in Paris, two hours from a normal local night. The half-hour version ends at 03:00 to 11:00 Paris time, four hours out. A 19:00 bed time is a real ask with a job and a family, which is what the half-hour column is for.
Westbound: London to New York, five hours behind
London time. One hour later per night.
| Night | Lights out | Alarm next morning | Half-hour version (bed / alarm) | Light the next day |
|---|---|---|---|---|
| Night 1 (four nights out) | 00:00 | 08:00 | 23:30 / 07:30 | Dim first hour after waking. Brightest light in the evening. |
| Night 2 | 01:00 | 09:00 | 00:00 / 08:00 | Same. |
| Night 3 | 02:00 | 10:00 | 00:30 / 08:30 | Same. Dark room or eye mask past sunrise. |
| Night 4 (night before the flight) | 03:00 | 11:00 | 01:00 / 09:00 | Same. |
03:00 to 11:00 in London is 22:00 to 06:00 in New York, one hour from a normal local night. The half-hour version ends at 20:00 to 04:00 New York time, three hours out.
Both offsets hold for most of the year but change for a few weeks around the daylight saving switches, because Europe and the US change clocks on different weekends. Check your dates on the world clock before building your own table. Why those gaps move is covered in time zones, UTC and DST explained.
When should you seek and avoid light?
This matters more than the bed time. The bed time is when you lie down. Light is what moves the clock.
The Yellow Book puts it plainly: bright light in the morning promotes phase advances, and evening light generally promotes phase delays.
Shifting earlier (eastbound). Get bright light after you wake, ideally outside, and keep evenings dim. Once the alarm is before sunrise there is no daylight, so bright indoor light is what you have. Research protocols used light boxes for those mornings, but the American Academy of Sleep Medicine's patient guidance on jet lag says to talk to your sleep provider before using light therapy, because light at the wrong point in the clock's cycle can push you further out of line.
Shifting later (westbound). Put your brightest light in the couple of hours before the new bed time and keep the morning dim. In the review's westbound example, the traveler avoids bright light after waking, wears very dark sunglasses if he must go out, and sleeps in a dark room or wears an eye mask.
The review also explains why the steps are small. If your light timing moves faster than your clock actually shifts, the light can land on the wrong side of the turning point and work against you.
What should you do on the flight itself?
Live on destination time from boarding. The Yellow Book advises timing meals, sleep and light to the destination time zone, staying hydrated and avoiding excess alcohol. The NHS adds: walk around the cabin, sleep if it is night at your destination, use an eye mask and earplugs if they help, and go easy on caffeine.
On the eastbound example, an evening departure from New York lands in the early Paris morning. If you ran the plan, you board near your new bed time, and Eastman and Burgess note that for a pre-shifted traveler the flight falls at a good circadian time to sleep. Take it. On the westbound example, a daytime flight is daytime in New York too: stay awake and eat on New York's schedule.
How do you lock the new schedule in on arrival?
Switch to local time completely on day one. The NHS guidance is to move to the new time zone's sleep schedule as quickly as possible, set an alarm so you do not oversleep, and get outside during the day.
Keep naps short. The CDC's traveler page suggests no more than 15 to 20 minutes, and the Yellow Book warns that lengthy daytime naps make night sleep harder. Why a long nap can also leave you feeling worse is covered in why you wake up groggy.
Keep the light rule for your direction. After a pre-shifted eastbound trip, Eastman and Burgess note that outdoor light after landing helps the clock advance rather than pulling it back. The CDC suggests caffeine and exercise for daytime alertness, avoiding both in the evening.
At the Yellow Book's average rates, the two hours left from the eastbound plan are roughly two days of adjustment instead of six, and the one hour left from the westbound plan is under a day.
Know where scheduling ends. Whether melatonin or sleep medication suits you, and at what timing, is a clinician's question. The AASM notes melatonin at the wrong time can worsen misalignment, and the NHS does not recommend it for jet lag, citing insufficient evidence. The AASM also suggests seeing a sleep doctor if you travel often and keep struggling.
Set the plan up now
Put each of your four alarm times into the sleep calculator in "I need to wake up at" mode. It counts back in 90-minute cycles plus about 15 minutes to fall asleep, so it suggests cycle-aligned bed times near the plan's: for eastbound Night 1's 06:00 alarm, five cycles lands on 22:15. The reasoning is in what time to go to bed to wake up at 6am.
Then add the four alarms on the online alarm clock, labelled Night 1 to Night 4. They are daily times with an on and off switch, so turn on only the one due next morning. The alarm rings from the open tab, so keep the device plugged in and awake, or copy the times into your phone.
FAQ
How many days does it take to adjust to a new time zone?
The AASM estimates about one day per time zone. The Yellow Book's average rates are about one hour a day after flying east and about 1.5 hours a day after flying west, so a six-zone eastbound trip takes close to a week without preparation and a westbound one settles faster. Shifting before you fly shrinks the hours left to close.
Should you shift your schedule before flying east?
Yes, if your week allows it. The CDC's traveler advice for eastbound trips is to go to bed an hour or two earlier than usual in the days before. Do not advance faster than an hour a day, and consider half an hour if you are a night owl. Trips of eight or more zones east need more care, because the clock can shift the wrong way.
Is it worth changing your watch on the plane?
As a cue, yes. The Yellow Book advises timing meals, sleep and light to the destination time zone, and that is easier when the time in front of you is already the destination's. The watch does nothing to your body clock itself; it changes when you choose to eat and sleep.
How long does jet lag last per time zone?
Roughly a day per zone crossed is the common estimate, and eastbound trips tend to take longer than westbound trips of the same size. The NHS says jet lag usually improves within a few days. If you fly often and keep struggling, the AASM suggests seeing a sleep doctor.
Work your target times out on the sleep calculator and set the four alarms now. This article is informational and is not medical advice.