Jet Lag: Circadian Resetting After Time Zone Travel
Jet lag resynchronizes at ~1 day per time zone eastward and 1.5 days westward; eastward travel is harder because the clock must advance against its natural ~24.2-hour free-running period.
| Measure | Value | Unit | Notes |
|---|---|---|---|
| Resynchronization rate (eastward) | ~1 | day per time zone | Clock must advance; harder due to 24.2h natural period |
| Resynchronization rate (westward) | ~1.5 | days per time zone | Clock must delay; easier but takes longer per zone in total days |
| Threshold for jet lag | ≥2 | time zones | Crossing 1 zone causes minimal symptoms; 2+ produces measurable impairment |
| Phase-shifting dose of melatonin | 0.5–3 | mg | Taken at destination bedtime; advances clock for eastward; lower dose equally effective |
| Athletic performance impairment | 5–12 | % reduction | Measured in professional athletes; reaction time and endurance affected |
Mechanism of Jet Lag
Jet lag (circadian dysrhythmia) occurs when rapid transmeridian travel displaces the body’s circadian clock from local environmental time. The biological clock, synchronized to the departure time zone, continues to drive sleep, wakefulness, hormone secretion, body temperature, and digestive function according to the previous time zone’s schedule — while the external environment (light, meals, social cues) follows the new time zone.
This misalignment produces the characteristic symptoms: insomnia at destination bedtime, daytime sleepiness, impaired concentration and reaction time, gastrointestinal disturbance, and general malaise.
Why Eastward Is Harder
The human circadian clock runs at approximately 24.2 hours in the absence of external time cues. This means:
- Natural drift: without entrainment, the clock delays by ~0.2h per day
- Westward travel (phase delay): aligns with the clock’s natural tendency; resynchronization ~1.5 days per time zone, but subjectively easier
- Eastward travel (phase advance): works against the natural drift; the SCN can typically advance only ~1 hour per day in response to morning light
For a New York → London flight (5 time zones east), typical full resynchronization takes 4–5 days. For London → New York (5 zones west), 6–8 days total but with milder symptoms throughout.
Light and Melatonin Strategies
Light exposure is the most powerful resetting tool:
- Eastward travel: seek bright morning light at destination; avoid evening light for first 2 days
- Westward travel: seek afternoon/evening light at destination; avoid bright morning light initially
Melatonin timing (phase-response curve):
- Eastward: take 0.5–1mg at destination bedtime for 2–4 nights
- Westward: some protocols use melatonin at destination waking time, though evidence is less robust
The phase-response curve (PRC) for melatonin, characterized by Lewy et al. (1992), shows melatonin advances the clock when taken in the morning (physiological dusk signal) and delays it when taken in the evening — the opposite of light’s PRC.
Athletic and Occupational Impact
Jet lag meaningfully impairs performance in professional athletes. Studies of NBA and NFL players show 5–12% performance reductions in teams traveling across 3+ time zones. Match results show west-coast teams playing in the east underperform their expected win probability, particularly in evening games. Airlines require mandatory recovery time before pilots can resume command duties after long eastward transits.
Related Pages
Sources
- Waterhouse J et al. — Jet lag: trends and coping strategies. Lancet (2007)
- Arendt J & Skene DJ — Melatonin as a chronobiotic. Sleep Med Rev (2005)
- Lewy AJ et al. — Melatonin shifts human circadian rhythms according to a phase-response curve. Chronobiol Int (1992)
- Eastman CI & Burgess HJ — How to travel the world without jet lag. Sleep Med Clin (2009)
Frequently Asked Questions
Why is eastward jet lag worse than westward?
The human circadian clock runs at ~24.2 hours — slightly longer than 24 hours. Westward travel requires the clock to delay (go later), which aligns with its natural tendency. Eastward travel requires advancing the clock (going earlier), which works against the natural drift. This asymmetry means most people can advance their clock by only about 1 hour per day, while delaying by 1.5 hours per day is easier.
Does melatonin help with jet lag?
Yes — melatonin is the most evidence-based pharmacological intervention for jet lag. Taking 0.5–3mg at the destination bedtime for 2–4 days after arrival accelerates circadian resynchronization. A Cochrane review (Herxheimer & Petrie, 2002) found melatonin effective for jet lag when crossing ≥5 time zones, particularly eastward travel.