Jet lag at an away race: why eastward is harder than westward

You flew in two days before the race, slept well, feel fine — and then out on the course your legs are empty and your heart rate has a life of its own. Sound familiar? The problem is that a subjective “I slept fine” and biological readiness after a flight are not the same thing. And the price you pay depends on which direction you flew.

Two different problems everyone confuses

Travel fatigue is the consequence of the journey itself: hours of immobility, dehydration in the dry cabin air, an awkward posture, the stress of connections, disrupted eating. It has nothing to do with time zones and clears up in a day or two. A ten-hour flight due south gives you travel fatigue and no jet lag at all.

Jet lag is a mismatch between your internal clock and local time. It appears only when you cross meridians, and it takes far longer to go away.

Telling them apart matters, because they're treated differently: fatigue with rest, jet lag with a phase shift through light and schedule.

Why eastward is harder

For most people the internal day is slightly longer than the astronomical one. So it's natural for the body to stretch the day — that is, to go to bed and get up later. That's exactly what happens when you fly west: you arrive, local evening comes “later” than your internal one, and adjusting is relatively easy.

Flying east demands the opposite — compressing the day, going to bed earlier than your body wants. That runs against the natural drift, and adaptation is noticeably slower.

The figures cited in reviews reflect this asymmetry directly: roughly one day of adaptation per hour of difference travelling east versus half a day per hour travelling west. The total duration of the after-effects ranges from 2–3 days to 8–10 days depending on the number of zones crossed.

The asymmetry shows up in results, too. An analysis of NBA games found that eastward jet lag in home teams was associated with a lower win share, smaller point differential, worse rebounding differential, and poorer shooting efficiency. Westward jet lag was not. So the effect isn't abstract: it can be read in the outcomes of professional games.

What actually breaks

The mechanisms are described in reasonable detail. A misaligned internal clock disrupts the daily body temperature rhythm, throws off the cortisol rhythm, and worsens sleep quality. Further down the chain come reduced central drive, worse motor unit recruitment, and slower reaction times.

For endurance athletes this means not so much a collapse in VO₂max as the fact that familiar work feels harder, and that holding pacing at threshold intensities becomes more difficult. On top of that, a shifted temperature rhythm hits thermoregulation — critical if the race is in the heat.

One separate detail: for most people peak performance falls in the second half of the day by internal time. A morning start after an eastward flight lands in your biological night twice over — by phase and by sleep debt.

How to plan

Count your arrival days by direction. Going east, budget about a day per zone; going west, half that. A flight six zones east with a race two days later is a race you're starting unprepared for.
Start the shift before you fly. For 3–4 days beforehand, move bedtime and wake-up by 30–60 minutes towards the destination time zone. You won't fully adjust, but you'll take the first slice of the work off the table.
Light is the main lever. Going east you need bright light in the morning by local time and avoidance of it in the evening; going west, the reverse. Getting the timing of light exposure wrong can shift your phase in the wrong direction.
Set your watch on the plane and live by destination time from that moment on — meals included.
First day: no key sessions. Light activity outdoors in daylight helps you adjust better than sleeping in a dark hotel room.
Don't schedule a test session right after landing. Its result will tell you about the flight, not about your fitness.
Budget for travel fatigue separately. Even a flight with no time-zone change costs a day of recovery.

Limitations

The bulk of the data is observational, and that has to be taken into account. The NBA analysis shows an association, but dozens of factors change for teams simultaneously: the schedule, the opponent, the travel, sleep in a hotel. Randomising flights among elite athletes is practically impossible, so the evidence base here is by definition weaker than in laboratory disciplines.

Between-person variation is large: chronotype, age, travel experience, and how strictly a person keeps to a schedule change the speed of adaptation several-fold. The “a day per zone” rule is an average guide, not a standard. And almost all of the numbers come from team sports rather than endurance — applying them to the marathon calls for a caveat.

Key points

Travel fatigue and jet lag are different things: the first clears in a day or two, the second drags on for up to 8–10 days.
Eastward is harder: about a day of adaptation per zone versus half a day going west — because compressing the day is harder than stretching it.
In the NBA, eastward jet lag was linked to worse results; westward was not.
Body temperature rhythm, cortisol rhythm, and sleep break down → central drive and reaction speed fall.
Plan your arrival by the direction of travel, start shifting your schedule 3–4 days out, and manage your light exposure.
First day: no key sessions; a test workout right after landing measures the flight, not your fitness.

Sources: “Do Long-Haul Travel and Jet Lag Affect Athletes' Physiological, Humoral and Performance Outcomes? A Systematic Narrative Review”, Sports, 2026;14(3):93. https://www.mdpi.com/2075-4663/14/3/93. “Eastward Jet Lag is Associated with Impaired Performance and Game Outcome in the National Basketball Association”. https://pmc.ncbi.nlm.nih.gov/articles/PMC9245584/. “Case Report: Jet lag and travel fatigue effects on performance in a world-class paralympic shot-putter after eastward long-haul transmeridian travel”, Frontiers in Sports and Active Living, 2026. https://doi.org/10.3389/fspor.2026.1745296