How Flying Works

Why Jet Lag Is Worse When You Fly East

In a sleep laboratory at Harvard in the 1990s, a team led by Charles Czeisler took away the one thing that keeps human days at 24 hours: daylight. Volunteers lived in rooms with no windows and no clocks, on an artificial schedule of 28-hour “days”. That schedule is too long for the body clock to follow, so it falls back to running at its own natural speed, and the researchers could measure it from hormone levels and body temperature.

Published in 1999, the result was strikingly consistent. Young and older volunteers alike had an internal day of about 24 hours and 11 minutes. Slightly longer than a real day. That small extra 11 minutes turns out to explain one of the most common complaints in travel: why flying east knocks you out more than flying west.

The clock in your head

Deep in the brain, a small cluster of nerve cells called the suprachiasmatic nucleus acts as the body’s master clock. It sets a daily rhythm, called a circadian rhythm (from Latin for “about a day”), for sleepiness, body temperature, hunger and hormones. Every morning, light reaching your eyes nudges it back into line with the sun.

Jet lag is what happens when you move faster than that nudge can keep up with. Land six time zones away and the local clock says morning while your body clock still says the middle of the night. You’re sleepy at lunch and wide awake at 3am until the clock catches up.

Interactive · your first day after landing

Fly east and the day shrinks. Fly west and it stretches.

6 time zones

Flying east across 6 zones, your first day is only 18 hours long. Your body clock has to advance: sleep and wake earlier than it wants to. Its natural day is already a little over 24 hours, so this is the harder direction.

Recovery, model estimate (Chaos, 2016)

6 zones

EASTmore than 8 days

WESTfewer than 6 days

9 zones

EASTnearly 2 weeks

WESTunder 8 days

From a mathematical model of the brain’s clock cells. Real recovery varies from person to person.

Why east is harder

Fly east and you lose hours. Your first day at the destination is shorter than 24 hours, and your body clock has to advance: fall asleep and wake up earlier than it wants to. Fly west and you gain hours. The day stretches, and your clock has to delay: stay up later.

Because the clock’s natural day is already a little longer than 24 hours, delaying comes more easily. It’s like a watch that runs slow: letting it fall further behind is easy, catching it up takes work. In 2016, physicists at the University of Maryland built a mathematical model of the brain’s clock cells and sent virtual travellers around the world. Their model took nearly two weeks to recover from nine time zones east, against under eight days for nine zones west.

Sunrise seen from an aircraft window above the clouds — Sunrise over the wing. On an eastbound overnight flight, morning comes hours earlier than your body expects. Photo: Wikimedia Commons (author and licence on the file page).

What actually helps

Light is the strongest signal the body clock gets, so timing it is the most useful thing you can do. The US CDC’s travel health guidance and sleep researchers give the same broad advice:

Flying east: get bright light in the morning at your destination and avoid it late in the evening. That pulls the clock earlier.
Flying west: get light in the late afternoon and evening. That pushes the clock later.
Switch to local time fast for meals and sleep, even if you don’t feel like it.
Choose arrival times with care. Landing in the morning after flying east gives you a full day of useful light.
Ask a doctor before using melatonin or sleeping pills. Timing matters, and they don’t suit everyone.

North–south flights are different. Fly from London to Johannesburg and you barely change time zone, so there’s no real jet lag, just the ordinary tiredness of a long night in a seat.

View from an aircraft cabin window at sunrise — Morning light through a cabin window. Bright light at the right time is the strongest reset signal the body clock gets. Photo: Wikimedia Commons (author and licence on the file page).

Sources & Further Reading

Common Questions

Frequently Asked Questions

Is jet lag worse flying east or west?

East, for most people. Flying east shortens your first day, so your body clock has to shift earlier. The clock naturally runs a little longer than 24 hours, so shifting later (after flying west) comes more easily than shifting earlier.

How long does jet lag last?

It depends on how many time zones you cross, which way you fly and the person. A 2016 modelling study from the University of Maryland estimated that recovering from nine time zones east could take nearly two weeks, against under eight days for the same trip west.

Do you get jet lag flying north or south?

Not real jet lag, because you stay in roughly the same time zone. You can still feel tired from a long flight, dry cabin air and broken sleep. That’s travel fatigue, and it passes after a good night’s sleep.

What is the fastest way to get over jet lag?

Light at the right time is the strongest tool. After flying east, get bright light in the morning at your destination; after flying west, get light in the evening. Shift your meals and sleep to local time quickly, and keep the first days light if you can.

Why does the body clock run longer than 24 hours?

Nobody fully knows why, but it has been measured. In a 1999 Harvard study, volunteers living on 28-hour days in a lab showed an average internal day of about 24 hours and 11 minutes. Daylight resets it a little every morning to keep it on 24 hours.

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Last reviewed: October 2026. Facts that change, such as fleets, airspace and airline ownership, are re-checked on a schedule. Spotted an error? Email contact@skyplus.news and we’ll fix it. See how we check facts.