Jet Lag: Why It’s Easier to Delay Your Body Clock Than Speed It Up

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Traveling across time zones can leave your body clock out of sync with the outside world, creating the well-known condition called jet lag. Jet lag is essentially a mismatch between your circadian rhythm—the body’s internal timekeeping system—and the local time in your new environment (Sack et al., 2007).

Our Natural Biological Clock

When humans are placed in environments without external time cues, such as natural light, our circadian rhythms tend to “free run.” In these conditions, researchers have found that the average human circadian cycle extends slightly longer than 24 hours, often between 24.2 and 25 hours (Aschoff & Wever, 1967). This explains why it is generally easier to extend our sleep–wake schedule to a later time than it is to force the body to wake and sleep earlier.

Why Traveling West Feels Easier Than Traveling East

Because our internal clock is slightly longer than 24 hours, delaying the circadian rhythm (as happens when traveling west) is more natural than advancing it (as happens when traveling east). Put differently, most people find it easier to stay up later than to fall asleep earlier (Eastman & Burgess, 2009).

How Much Can We Shift Each Day?

Research shows that circadian rhythms can realistically shift by about one hour per day with the right environmental cues (Eastman & Burgess, 2009). For example, in a study where participants tried to shift their schedule before an eastward flight, advancing bedtime by one hour per day combined with morning light exposure led to smoother adjustment. When researchers attempted to push participants forward by two hours per day, the changes proved too extreme, resulting in sleep difficulties and incomplete adaptation (Burgess et al., 2003).

Light as the Most Powerful Cue

Light exposure is the strongest factor that regulates the circadian clock. Morning light tends to advance the circadian rhythm, which is helpful for adjusting to eastward travel, while evening light delays the rhythm, which is useful for westward travel (Czeisler et al., 1989). This is why jet lag strategies often focus on carefully timed light exposure to help reset the internal clock.

Practical Takeaways

If you are traveling across time zones, remember that:

  • Your body clock adjusts slowly, usually about one hour per day.

  • Traveling west is easier than traveling east because delaying your clock fits better with your biology.

  • Strategic exposure to light at the right times can help shift your circadian rhythm more efficiently.

  • Preparing in advance by gradually shifting your schedule before departure can reduce jet lag symptoms.

References:

Aschoff J. (1967). Human circadian rhythms in activity, body temperature and other functions. Life sciences and space research, 5, 159–173.

Burgess, H. J., Crowley, S. J., Gazda, C. J., Fogg, L. F., & Eastman, C. I. (2003). Preflight adjustment to eastward travel: 3 days of advancing sleep with and without morning bright light. Journal of biological rhythms, 18(4), 318–328. https://doi.org/10.1177/0748730403253585

Czeisler, C. A., Allan, J. S., Strogatz, S. H., Ronda, J. M., Sánchez, R., Ríos, C. D., Freitag, W. O., Richardson, G. S., & Kronauer, R. E. (1986). Bright light resets the human circadian pacemaker independent of the timing of the sleep-wake cycle. Science (New York, N.Y.), 233(4764), 667–671. https://doi.org/10.1126/science.3726555

Eastman, C. I., & Burgess, H. J. (2009). How To Travel the World Without Jet lag. Sleep medicine clinics, 4(2), 241–255. https://doi.org/10.1016/j.jsmc.2009.02.006

Sack, R. L., Auckley, D., Auger, R. R., Carskadon, M. A., Wright, K. P., Jr, Vitiello, M. V., Zhdanova, I. V., & American Academy of Sleep Medicine (2007). Circadian rhythm sleep disorders: part I, basic principles, shift work and jet lag disorders. An American Academy of Sleep Medicine review. Sleep, 30(11), 1460–1483. https://doi.org/10.1093/sleep/30.11.1460

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