Why jet lag hits harder flying east – and how to avoid the slump

Anyone who flies long-haul regularly knows the eastbound leg is the rough one. And the reason is how your body clock runs.

It may surprise you, but the human body clock does not run on a 24-hour cycle. It runs to about 24 hours and 11 minutes, so it drifts slightly later every day.

Flying west means shifting your clock later, which your body finds easier. But going east means shifting it earlier, and that’s harder to manage.

In this guide, we explain the science behind jetlag and some actionable strategies to make eastern travel easier.

Key takeaways

• The human body clock runs to about 24 hours and 11 minutes, so it naturally drifts later each day.
• Flying east means shifting earlier, which works against it.
• Light resets your clock so you can use it to manage jetlag.

Why east is harder than west

The intrinsic period of the human clock was measured precisely in 1999 , in young and older adults living under carefully controlled lighting. It averaged 24 hours and 11 minutes.

That subtle pull of your body clock towards a later bedtime is why staying up is easier than getting up early, and why eastbound travel takes longer to recover from.

Flying east requires your clock to advance, and flying west requires it to delay . Your longer-than-24-hour-cycle means your body is always trying to delay, so part of a westbound shift happens without you doing anything.

If you land in Europe from the US and spend four days wide awake at 2am and dragging yourself out of bed at 9, that’s your body clock still running on home time.

Stop your clock going the wrong way

Light shifts your clock differently depending on when you get it. The switch happens around your temperature minina, which is a few hours before you normally wake. Light after that point moves your clock earlier . Light before it moves your clock later.

When you fly east, it can be a trap. Your body is still on home time when you land, so bright local morning light can hit before your body’s temperature minma switchover point, so your clock is pushed later, exactly when you need it earlier.

Scientists call this antidromic re-entrainment: the clock adjusting the long way round.

In an example from a widely cited review of jet lag , a San Francisco–Beijing round trip across nine time zones took nine days to adjust heading west and ten coming back east, because the clock went the wrong way.

So morning light isn’t automatically good advice. After a big eastward flight, skipping early sun for the first few days and getting your light in late morning instead helped shift your clock more efficiently.

How to adjust faster flying east

You need to advance your clock, which means light later in your body’s morning and darkness in your body’s evening.

• Shift your sleep earlier before you go. Move bedtime and wake time 30 to 60 minutes earlier each day for two or three days before the flight.
• Get bright light in the mid-to-late morning at your destination, not at dawn. If you land with a large time difference, wait until mid-morning local time before seeking strong light.
• Wear dark sunglasses if you are out early in the first couple of days.
• Keep the evenings dim and go to bed at local time even if you are not tired.
• Get outside properly once you are adjusting. Once your clock has caught up, normal morning light keeps it there.

How to adjust faster when flying west

This direction is easier. You need to delay your clock rather than advance it.

• Stay up later for a night or two before you travel, which is easier than getting up early.
• Seek light in the local evening. That moves your clock later, and it is when you most want to be awake anyway.
• Avoid bright light early in the local morning for the first day or two.
• Do not nap late in the local afternoon, which makes the evening harder.

Frequently asked questions

How long does jet lag last?

It depends on the number of time zones and the direction. Eastbound takes longer than westbound for the same distance. In one worked example across nine zones, full adjustment took nine days going west and ten coming back east.

Is there any point adjusting for a short trip?

Often not. For a trip of two or three days, staying on home time where your schedule allows it avoids two shifts instead of one. That is worth more than half-adjusting in both directions.

Does melatonin help?

Reviews of jet lag interventions  cover melatonin alongside light, exercise and meal timing. Timing matters as much as dose, and both are worth discussing with a doctor or pharmacist rather than guessing.

Why do I feel worse flying east even on a short hop?

Because direction matters as well as distance. Your clock has to advance rather than delay, and advancing works against its natural drift of eleven minutes a day.

References

1. Czeisler CA, Duffy JF, Shanahan TL, et al. Stability, precision, and near-24-hour period of the human circadian pacemaker. Science 1999;284(5423):2177–2181. PMID 10381883 . Intrinsic period averaged 24.18 hours.
2. Eastman CI, Burgess HJ. How to travel the world without jet lag. Sleep Medicine Clinics 2009;4(2):241–255. PMID 20204161
3. Khalsa SB, Jewett ME, Cajochen C, Czeisler CA. A phase response curve to single bright light pulses in human subjects. Journal of Physiology 2003;549(3):945–952. PMID 12717008
4. Forbes-Robertson S, Dudley E, Vadgama P, et al. Circadian disruption and remedial interventions: effects and interventions for jet lag for athletic peak performance. Sports Medicine 2012;42(3):185–208. PMID 22299812