Eastbound vs Westbound Travel
Introduction
Not all jet lag is equal. Crossing the same number of time zones produces noticeably different recovery times depending on which direction you travel. Eastbound travelers typically adapt more slowly, experience more fragmented sleep, and face a greater risk of the clock shifting in the wrong direction. The reason lies in a fundamental property of the human circadian system: its natural period is slightly longer than 24 hours.
The Human Circadian Period
In the absence of external time cues, no sunlight, no clocks, no social schedule, the human circadian clock runs on a cycle of approximately 24.2 hours, a few minutes longer than the solar day (Czeisler et al., 1999). This means the clock has a built-in tendency to drift later. Without environmental signals to correct it, most people would naturally go to bed and wake up a little later each day.
This slight asymmetry has a direct consequence for travel. Westbound travel requires delaying the clock, pushing sleep and wake times later. This aligns with the clock’s natural drift, so the body cooperates. Eastbound travel requires advancing the clock, shifting sleep and wake times earlier. This opposes the clock’s natural tendency, and the body resists.
Why Delaying Is Easier
Laboratory studies confirm what the biology predicts. In a controlled experiment measuring the maximum daily shift achievable under optimized light and melatonin protocols, subjects shifted their circadian clock by 9.6 hours in four days in the delay direction versus only 6.2 hours in four days in the advance direction (Eastman & Burgess, 2009). Delaying is approximately 55% faster under optimal conditions.
The physiological basis is the shape of the phase response curve, the map of how the clock responds to light at different times of day. The delay region of the curve, triggered by light in the biological evening, is broader and produces larger per-hour shifts than the advance region, which is activated only during a narrower window in the biological morning (Khalsa et al., 2003).
Eastbound Challenges
Eastbound travel requires advancing the circadian clock, which works against its natural drift. Several difficulties compound this:
Slower adaptation, Because advancing the clock is intrinsically harder, the circadian system takes longer to align with the destination time zone. Each day’s progress is smaller, and total recovery takes more days.
The risk of wrong-way shifting, For very large eastward crossings (typically eight or more time zones), the clock may find it easier to delay all the way around the 24-hour cycle rather than advance the shorter apparent distance. This phenomenon is called antidromic re-entrainment, the clock travels the “long way around” and takes far longer to adapt. In a study of 8 subjects crossing 11 time zones eastward, 7 out of 8 adapted by delaying rather than advancing (Takahashi et al., 2001). Their clocks went in the opposite direction from what was intended.
Difficulty falling asleep, After an eastbound flight, the traveler’s biological night still corresponds to the afternoon at the origin. When they go to bed at the destination’s local bedtime, the circadian system is not yet ready for sleep. This produces difficulty falling asleep in the early part of the night, and often early awakening before the local alarm time.
Counterproductive sunlight, Morning light at the destination arrives at a time that corresponds to the biological evening of the origin. Depending on the size of the crossing, this local “morning” light may fall in the delay zone of the phase response curve, actively pushing the clock further away from the destination’s time zone rather than toward it (Eastman & Burgess, 2009). This makes unguided sunlight exposure after eastbound travel potentially counterproductive.
Westbound Challenges
Westbound travel aligns with the clock’s natural drift, making adaptation intrinsically easier. However, it is not without difficulties:
Early morning awakening, After a westbound flight, the traveler’s biological wake time corresponds to the early hours of the morning at the destination. When they try to stay in bed until the local wake time, the circadian system has already started promoting alertness. The result: waking too early, difficulty returning to sleep, and fatigue concentrated in the late afternoon.
Evening sleepiness, The traveler’s biological bedtime falls in the early evening of the destination. Many travelers feel a strong urge to sleep several hours before the local bedtime, and falling asleep prematurely makes it harder to shift the clock by the full required amount.
Gradual completion, While partial adaptation happens quickly, full adaptation to large westward crossings still requires several days of progressive delay. Without managing light and sleep timing, the clock may partially adapt and then stall.
Typical Adjustment Speeds
The following table summarizes expected adaptation timelines with a combined light and melatonin protocol, based on Roach & Sargent (2019):
| Shift | Partial adaptation | Complete adaptation |
|---|---|---|
| 3 h west | Day 1 | Day 4 |
| 6 h west | Day 2 | Day 6 |
| 9 h west | Day 3 | Day 7 |
| 3 h east | Day 1 | Day 4 |
| 6 h east | Day 3 | Day 6 |
| 9 h east | Day 5 | Day 8 |
The asymmetry is most visible at larger crossing sizes. A 9-hour westward shift reaches partial adaptation two days earlier than a 9-hour eastward shift, and complete adaptation one day earlier.
Implications for Trip Planning
Pre-flight shifting, Gradually shifting sleep and light exposure toward the destination time zone before departure is beneficial for any trip, but it is especially valuable before eastward travel, where adaptation at the destination is slower. Even one or two days of pre-departure advancing can meaningfully reduce jet lag severity (Eastman et al., 2005).
Large eastward crossings, consider delaying instead, For crossings of eight or more time zones eastward, the distance the clock needs to travel by advancing may actually be longer than the distance available by delaying in the opposite direction. In these cases, deliberately delaying the clock the “other way around” may be faster and more physiologically natural than attempting to advance. This is a strategy to discuss with any structured jet lag plan.
Chronotype effects, People who are naturally late sleepers (“night owls”) have a longer free-running period than early risers, making them even better suited to delay-based adaptation. Eastman and Burgess (2009) note that night owls may benefit from a delaying strategy even for moderate eastward trips where early risers would advance.
Light management is direction-sensitive, Because local sunrise lands at very different positions on the circadian clock for eastbound versus westbound travelers, the timing of light-seeking and light-avoidance must be calibrated to both the direction and magnitude of travel. Generic advice to “get morning sunlight” can help or hurt depending on how many time zones were crossed and in which direction.
Key Takeaways
- The human circadian clock has a natural period of ~24.2 hours, creating a built-in bias toward delay.
- Delaying the clock is physiologically easier: ~9.6 hours in four days versus ~6.2 hours for advancing under optimized conditions.
- Eastbound travel (requiring the clock to advance) is harder: adaptation is slower, wrong-way shifting is possible for large crossings, difficulty falling asleep is common, and local morning light may oppose adaptation.
- Westbound travel (requiring the clock to delay) is faster but still produces early awakening and premature evening sleepiness.
- For crossings of 8 or more time zones eastward, delaying all the way around may be the better strategy.
- Pre-flight shifting is especially valuable before eastbound trips.
- Light management must be direction-specific; generic sunlight advice can backfire after eastbound travel.
References
- Czeisler, C. A., Duffy, J. F., Shanahan, T. L., Brown, E. N., Mitchell, J. F., Rimmer, D. W., … Ronda, J. M. (1999). Stability, precision, and near-24-hour period of the human circadian pacemaker. Science, 284(5423), 2177–2181.
- Eastman, C. I., & Burgess, H. J. (2009). How to travel the world without jet lag. Sleep Medicine Clinics, 4(2), 241–255.
- Eastman, C. I., Gazda, C. J., Burgess, H. J., Crowley, S. J., & Fogg, L. F. (2005). Advancing circadian rhythms before eastward flight: A strategy to prevent or reduce jet lag. Sleep, 28(1), 33–44.
- Khalsa, S. B. S., Jewett, M. E., Cajochen, C., & Czeisler, C. A. (2003). A phase response curve to single bright light pulses in human subjects. Journal of Physiology, 549(3), 945–952.
- Roach, G. D., & Sargent, C. (2019). Interventions to minimize jet lag after westward and eastward flight. Frontiers in Physiology, 10, 927.
- Sack, R. L. (2010). Jet lag. New England Journal of Medicine, 362(5), 440–447.
- Takahashi, T., Sasaki, M., Itoh, H., Ozone, M., Yamadera, W., Hayashida, K., … Shibui, K. (2001). Re-entrainment of the circadian rhythm of plasma melatonin in an 11-h eastward bound flight. Psychiatry and Clinical Neurosciences, 55(3), 275–276.