Is jet lag worse flying east or west? Here's what the science says

Yes. Is jet lag worse flying east or west is settled science: flying east is harder on the body than flying the same distance west, because eastward travel forces your internal clock to advance, sleeping and waking earlier, instead of delay, sleeping and waking later, and the body resists advancing far more than it resists delaying.

Is jet lag worse flying east or west: what the evidence shows

Travelers have argued about this for decades, usually based on one bad trip. Airlines and travel blogs repeat the eastbound-is-worse line so often it has become background noise, rarely explained and rarely backed by a number. Sleep scientists, though, have settled this one. A widely cited chronobiology review by Charmane Eastman and Helen Burgess, published in Sleep Medicine Clinics, states plainly that it is much more difficult to phase advance the human circadian clock than to phase delay it. Flying east demands an advance. Flying west only asks for a delay.

The mechanism sits in how the body clock naturally runs. Left alone, with no alarm clock or daylight cues, the human circadian rhythm cycles slightly longer than 24 hours, closer to 24 hours and 11 minutes on average. That means the body already drifts toward a later bedtime every single day. Westward travel works with that drift. Eastward travel works against it, asking the clock to do the one thing it resists: move earlier. Horacio de la Iglesia, a biologist at the University of Washington, has noted that the exact mechanism behind this asymmetry is still not fully understood, but it almost certainly reflects that basic difficulty of advancing rather than delaying the clock.

The 2016 study that put a number on it

In 2016, a team at the University of Maryland, led by physicist Michelle Girvan along with Zhixin Lu, Kevin Klein-Cardena, Steven Lee, Thomas Antonsen Jr. and Edward Ott, built a mathematical model of the suprachiasmatic nucleus, the cluster of pacemaker cells in the brain that keeps the body's master clock, and published it in the journal Chaos. They modeled those cells as thousands of coupled oscillators and ran simulated flights across different numbers of time zones in both directions.

The results matched what clinicians already suspected, but with harder edges. Crossing three time zones east took a little over four days to resync, against under four days going the same three zones west. At six zones east, recovery passed eight days. At nine zones east, more than twelve days, compared with under eight days for nine zones west. Oddly, by twelve zones the two directions converge to roughly nine days each, because past that point flying further east becomes mathematically the same clock shift as flying less far in the other direction. A long eastbound trip and a shorter one can land you in a worse spot than a longer westbound trip, which is the opposite of what most travelers expect.

The team is careful about what the model can claim. Girvan has said the gap between a person's own natural rhythm and 24 hours controls how badly they experience jet lag, and some people run closer to 24.5 hours than others, which shifts these numbers around individually. STAT News also reported that practicing jet lag researchers do not think you need oscillator physics to explain the east-west gap at all. The simpler story, that advancing is harder than delaying, already accounts for most of it. Treat the Maryland numbers as a useful estimate from a simplified model, not a clinical measurement of any one traveler.

Why advancing your clock is harder than delaying it

The clearest human data comes from Eastman and Burgess's review of real travelers rather than models. On average, people's circadian clocks delayed about 92 minutes per day after flying west, but advanced only about 57 minutes per day after flying east, roughly 60 percent slower in the harder direction. That gap compounds across a trip. A six-zone westward flight can be absorbed in three or four days. The same six zones eastward can take a week or more, because each day only closes about two thirds of the gap a day going west would close.

Light exposure makes the asymmetry worse if you get it wrong. Dr. Charles Czeisler, a circadian rhythms researcher at Brigham and Women's Hospital, has pointed out that flying east means your body meets morning light in the new location at roughly midnight by its old clock, a strong signal sent at exactly the wrong moment. On trips of eight or more time zones east, Eastman and Burgess note the clock can even shift in the wrong direction entirely, a phenomenon called antidromic re-entrainment, which is part of why very long eastbound hauls feel disproportionately brutal compared with the same distance flown west.

How many days you will actually lose, by time zones crossed

Put together, the practical numbers look like this. A short eastbound hop of two or three zones, New York to London territory, costs roughly four days of adjustment. A six-zone eastbound trip, New York to much of Western Europe at certain times of year or the US to the Middle East, pushes past eight days. A nine-zone eastbound trip, the US West Coast to much of Europe, runs over twelve days, more than the length of many vacations. The same distances westward come in meaningfully lower: under eight days for nine zones, under four for three.

This cuts against the old rule of thumb that you lose roughly one day of adjustment per time zone crossed. The Maryland model found that rule does not hold: it underestimates long eastbound trips and overestimates some long westbound ones. The honest planning assumption is that eastbound costs noticeably more days than the zone count alone suggests, and the gap widens up to about the nine-zone mark.

Light and melatonin timing that actually help

Eastman and Burgess's clinical guidance comes down to timing light and melatonin against your body's temperature minimum, the point in the night your core temperature bottoms out. Light after that point and in the new morning advances the clock, which is what eastbound travelers need. Light before that point, in the evening, delays it, which is what westbound travelers need. In practice: seek bright light outdoors soon after waking on an eastbound trip, and avoid it in the evening hours before departure if you can start shifting early; do the opposite for a westbound trip, with evening light and reduced morning brightness.

Melatonin works the same way in reverse of what people assume. Taken in the afternoon, well before your normal bedtime, it produces the biggest advances, helpful for eastbound travel. Taken in the morning, it produces delays, helpful westbound. Eastman and Burgess's working estimates use low doses, around half a milligram, timed a couple of hours before your body's natural melatonin onset, specifically because larger doses taken at the wrong time can sedate without shifting the clock in the direction you need. None of this works instantly. It nudges the clock a bit faster each day rather than resetting it on arrival, and arriving already sleep-deprived makes everything worse, so getting how much sleep you need sorted out before a long trip matters as much as anything you do mid-flight.

Who gets hit hardest, and when the pattern flips

Not everyone experiences this identically. People whose natural circadian period runs further from 24 hours, which the Maryland model flags as the single biggest individual variable, tend to feel eastbound jet lag more acutely. Eastman and Burgess also note that committed night owls sometimes do better delaying their sleep further before an eastbound flight rather than trying to advance it, essentially taking the long way around the clock, which can beat fighting the natural drift head-on.

None of this is a guarantee. Dehydration, alcohol, cramped sleep on the plane and plain travel stress pile on top of the circadian effect and vary trip to trip, which is why some travelers swear westbound hits them harder despite the population-level data. If you are already managing the general discomforts of a long-haul flight, sorting out something like compression socks for long flights is a different problem than jet lag, but it belongs on the same pre-flight checklist. The east-west asymmetry is real and documented. It is not destiny for every individual trip.

Our take

The claim holds up. Flying east really is harder on the body than flying the same number of time zones west, for a reason sleep scientists can explain and a 2016 physics model has put real numbers on. Expect roughly 50 percent more recovery time eastbound than the same distance westbound, more for very long hauls, and use morning light and well-timed, low-dose melatonin to close the gap faster rather than just waiting it out.

This is general information, not medical advice. Talk to a clinician before changing anything, especially if you are pregnant, nursing, or managing a condition.