Primary sources · 4
- [1] Sack (2010) — Jet lag review — clinical chronobiology, recovery rates, light and melatonin interventions · New England Journal of Medicine 362:440-447 · February 2010 https://www.nejm.org/doi/full/10.1056/NEJMcp0909838
- [2] Lu et al. (2016) — Mathematical model that explains the east-west asymmetry of jet-lag recovery — Chaos journal · Chaos 26:094811 · September 2016 https://doi.org/10.1063/1.4954275
- [3] Czeisler et al. (1999) — Foundational paper measuring the free-running human circadian period at 24.18 h · Science 284:2177-2181 · June 1999 https://www.science.org/doi/10.1126/science.284.5423.2177
- [4] AASM clinical practice guidelines — Evidence-based recommendations for jet-lag treatment, including timed light and melatonin · American Academy of Sleep Medicine · Various editions https://aasm.org
Jet lag is your body's circadian clock disagreeing with the local light cycle. The clock can shift only so fast — about 92 minutes per day when the trip is westward, and only 57 minutes per day when eastward — which is why a Tokyo → London trip feels like one easy day and London → Tokyo feels like a brutal four.
The clock runs slow by design
In 1999 the Czeisler group measured the free-running period of the human circadian clock — the cycle the body produces when isolated from light cues — at 24.18 hours, slightly longer than a solar day. Morning sunlight each day advances the clock by the missing 11 minutes to keep it in sync. This small bias has a large consequence: shifting the clock to align with a later local time (westward travel) is easy, shifting it earlier (eastward) is hard.
Eastward is roughly 1.6 × harder than westward
The simple division — 92 / 57 ≈ 1.6 — gives the recovery-time asymmetry. A 6-hour westward flight (say New York → Lisbon) takes about 4 days to fully resynchronise; the same 6-hour eastward (Lisbon → New York) takes about 6.3 days. The asymmetry shows up consistently in sleep-lab measurements and in published clinical reviews.
The 12-hour case rejoins the directions because at the antipode the phase shift wraps — the body can resynchronise either by 12-hour advance or by 12-hour delay. Most people end up doing the easier delay, which is why a Tokyo → London or London → Tokyo trip feels roughly equally hard in either direction, around 7–8 days to full reset.
AirMilesCalc's jet-lag bands
We split the absolute hour difference between origin and destination into four severity bands and give recovery estimates for each. The bands match the Sack 2010 clinical framework reasonably well, though the published rates only specify the average — individual variation is large (some travelers reset in half the time, some in twice).
| |hours shift| | Severity | Westward recovery | Eastward recovery |
|---|---|---|---|
| ≤ 2 | None | 0 days | 0 days |
| 3 – 5 | Mild | 2 – 3 days | 3 – 5 days |
| 6 – 9 | Moderate | 4 – 6 days | 6 – 10 days |
| 10+ | Severe | 6 – 8 days | 10+ days (often delay-strategy) |
What actually helps
Three interventions have rigorous evidence. Timed light exposure is the most powerful — morning bright light advances the clock for eastward trips; evening bright light delays it for westward. Strategic melatonin (0.5–3 mg, taken 30 minutes before target local bedtime) accelerates the shift modestly. Sleep-schedule pre-adaptation starting 2–3 days before departure can pre-load some of the shift. What does not help: in-flight diet adjustments, anti-jet-lag supplements other than melatonin, and large amounts of caffeine.