Why Flying East Causes Worse Jet Lag Than Flying West

Travel

Cross six time zones westward and you will probably feel normal in four days. Cross the same six eastward and it can take a week. The asymmetry is real, it is measurable, and it comes down to one biological fact: the human body clock does not run at exactly 24 hours. It runs slightly longer — which makes stretching a day easy and compressing one genuinely hard.

Key takeaways

The internal circadian period averages about 24 hours and 11 minutes, not 24 hours.
Flying west asks for a phase delay (go to bed later) — the direction your clock already wants to drift.
Flying east asks for a phase advance (go to bed earlier) — against the drift.
Rough recovery: about 1 time zone per day going east, and 1.5 per day going west.
The same light exposure can shift you forwards or backwards depending on when you get it, relative to your body temperature minimum.
Beyond about 10 zones eastward, it is often faster to treat the trip as a westward shift instead.

Your Body Clock Runs Slow

In the late 1990s researchers at Harvard used a protocol called forced desynchrony, in which volunteers lived for weeks on artificial day lengths with no time cues, to measure the free-running period of the human circadian pacemaker. The answer, published in Science in 1999, was about 24.18 hours — 24 hours and 11 minutes — and remarkably consistent between individuals.

That eleven-minute surplus has to be corrected every single day, and morning light does the correcting. It also means your clock has a built-in preference. Left alone, it drifts later: this is why almost everyone finds it easier to stay up an extra hour than to fall asleep an hour early, why Sunday nights are harder than Friday nights, and why the spring clock change costs more sleep than the autumn one.

A westward flight asks you to do the thing your body was already inclined to do. An eastward flight asks you to fight it.

Phase Delay Versus Phase Advance

	Flying west	Flying east
Example route	London → New York	New York → London
What the day does	Gets longer	Gets shorter
What your clock must do	Phase delay — shift later	Phase advance — shift earlier
Matches natural drift?	Yes	No
Typical symptom	Waking at 4:00 AM, fading in the evening	Lying awake at midnight, unable to get up
Typical recovery	About 1.5 zones per day	About 1 zone per day

Put concrete numbers on it. London to New York is five zones westward: expect roughly three to four days. New York to London is the same five zones eastward: expect five to six. On a longer route the divergence widens. Los Angeles to Tokyo, sixteen or seventeen hours of clock change, is one of the hardest trips in commercial aviation in one direction and unremarkable in the other — see London vs Tokyo and New York vs New Delhi for two heavily travelled eastward shifts.

The Pivot Point Almost Nobody Is Told About

Everyone knows light resets the body clock. What is rarely explained is that light does not always push in the same direction — it pushes forwards or backwards depending on when you receive it, and the hinge is your core body temperature minimum, usually written as Tmin.

Tmin falls roughly two hours before your habitual wake time. If you normally wake at 7:00 AM, your Tmin is around 5:00 AM. The rule is:

Light in the hours before Tmin delays your clock — it pushes you later, which is what you want when flying west.
Light in the hours after Tmin advances your clock — it pulls you earlier, which is what you want when flying east.

This is where eastbound travellers routinely make things worse. Fly New York to London overnight, land at 7:00 AM local, and step into bright morning sunshine. London’s 7:00 AM is 2:00 AM by your body clock — three hours before your Tmin. That sunshine is not advancing you towards London time; it is delaying you further into New York time, exactly the wrong direction.

The fix costs nothing. On the morning you land after an eastward flight of five hours or more, wear sunglasses and stay out of direct sun until roughly mid-morning local time, then get outside deliberately. Each subsequent day your Tmin moves about an hour earlier, so you can start seeking light an hour earlier too:

Day in London (after New York, +5 zones)	Avoid bright light until	Then seek daylight
Arrival day	about 10:00	10:00–13:00
Day 2	about 09:00	09:00–12:00
Day 3	about 08:00	08:00–11:00
Day 4	no restriction	normal morning light

Westbound the problem inverts and is much easier to manage: seek afternoon and early-evening light, and avoid early-morning light for the first day or two so you are not pulled back towards home time. The general light strategy is summarised in How to Beat Jet Lag, and the pre-flight preparation that shrinks the whole problem in Best Sleep Strategies Before Long Flights.

When East Is So Far East That It Becomes West

There is a threshold past which the advice reverses. An eastward shift of N hours is mathematically the same destination as a westward shift of 24 minus N hours — and since your body finds delays easier than advances, at some point the long way round is quicker.

Cross ten or more zones eastward and the circadian system frequently resynchronises the “wrong” way on its own, delaying rather than advancing. Researchers call this antidromic re-entrainment, and it explains the peculiarly long, disorderly jet lag people report on routes such as New York to Tokyo or London to Auckland: the body is trying to travel one way while the traveller assumes it is travelling the other.

Route	Nominal shift	Best strategy
New York → London	+5 east	Advance: early nights, late-morning light
London → Dubai	+3 east	Advance; minor adjustment
New York → New Delhi	+9:30 east	Borderline; advance if staying a week or more
Los Angeles → Tokyo	+16 east (= 8 west)	Delay: stay up late, evening light
London → Sydney	+9 to +11 east	Delay is usually easier over long stays

Trans-Pacific routes add a second complication, because they also cross the International Date Line and change the calendar date. Always compute your adjustment from the number of hours shifted, never from the dates printed on the ticket — the reasoning is set out in Understanding Flight Times Across Time Zones, the practical consequences in Crossing the International Date Line, and the most extreme gaps in The Largest Time Difference on Earth.

The Evidence from Professional Sport

Sports leagues are an unusually clean natural experiment: thousands of matches, standardised conditions, and teams flying in both directions on tight schedules. A 2017 analysis in the Proceedings of the National Academy of Sciences examined twenty seasons of Major League Baseball and found the effect clearly.

Eastward travel produced measurable performance losses that westward travel of the same distance did not. The striking finding was that a jet-lagged home team — one returning east from a road trip — lost much of its home-field advantage, and that pitchers on eastward-lagged teams gave up noticeably more home runs. Comparable studies in the NBA and NFL have found similar directional asymmetries.

The practical lesson for travellers is the same one professional teams act on: if something important happens shortly after an eastward flight, arrive earlier than feels necessary. A day of margin buys back more than any in-flight routine.

A Direction-Specific Checklist

Step	Flying east	Flying west
Days before departure	Sleep and wake 30–60 min earlier each day	Sleep and wake 30–60 min later each day
On the aircraft	Sleep as early in the flight as possible	Stay awake; sleep late in the flight
Arrival morning	Sunglasses until mid-morning local	Get outside as soon as convenient
Arrival evening	Dim light, screens away, early night	Bright light and activity into the evening
Caffeine	Morning only, never after midday local	Useful in the late afternoon on day 1
Expect to feel normal in	Roughly one day per zone	Roughly two days per three zones

One more consolation: a purely north–south flight, however long, crosses few or no zones and produces fatigue rather than true jet lag. And a domestic hop like New York to Los Angeles is only three zones in either direction — enough to notice, not enough to plan around. How zone boundaries map onto longitude is covered in How Time Zones Work Around the World.

Frequently Asked Questions

Why is flying east worse for jet lag than flying west?

Because the human body clock has a natural period of about 24 hours and 11 minutes, so it drifts later on its own. Flying west asks it to shift later, which matches that drift. Flying east asks it to shift earlier, which works against it, and the adjustment therefore takes noticeably longer.

How much longer does eastward jet lag last?

As a rule of thumb, recovery runs at roughly one time zone per day going east and about one and a half zones per day going west. A six-zone trip therefore means about six days eastbound and four westbound, though individuals vary widely.

Should I get morning sunlight after an eastward flight?

Not immediately. If you land early in the morning after crossing five or more zones eastward, local sunrise falls before your body temperature minimum, and light at that point pushes your clock the wrong way. Wear sunglasses until around mid-morning local time on the first day, then seek daylight.

Is it true that for very long eastward trips you should adjust westward instead?

Often, yes. An eastward shift of more than about ten hours is equivalent to a smaller westward shift, and since delays are easier than advances, deliberately staying up late and seeking evening light can resynchronise you faster. Los Angeles to Tokyo, a nominal 16 hours east, is usually handled as an 8-hour westward shift.

Does the direction still matter for short flights?

Below about three zones the asymmetry is small enough that most people will not notice it, and a purely north–south flight produces travel fatigue rather than jet lag. The direction effect becomes the dominant factor from roughly four zones upward.