Days needed to recover from jet lag based on time-zones crossed and direction.
Recovery ≈ time-zones × direction-factor × age-factor. Eastbound: ~1 day per timezone (forcing earlier sleep). Westbound: ~0.66 day per timezone (delaying sleep is easier). Add 5 % per decade above age 25 for slower circadian re-entrainment.
Jet lag is the desync between your circadian clock and the local time after crossing time zones. The clock takes days to re-entrain; in the meantime sleep is fragmented, alertness drops, digestion is irregular, and athletic or cognitive performance can fall by 10–30 %. The asymmetry — east-bound (advancing the clock, sleeping earlier) is harder than west-bound (delaying, staying up later) — is well-documented but consistently underestimated. A typical heuristic from sleep research: 1 day per timezone for east-bound, 0.66 days per timezone for west-bound, with age penalties of ~5 % per decade beyond 25. This calculator translates origin and destination UTC offsets into a recovery-day estimate, flags trips too short for full adaptation (where partial-shift staying-on-home-time strategies might be better), and visualizes the timeline.
Time-zones crossed = |destination_UTC − origin_UTC|. The calc accepts half-hour offsets (some timezones use UTC+5:30, +5:45, +9:30, etc.). The direction is taken from the offsets if "auto", or forced via the dropdown.
Recovery rate: - East-bound: 1.0 day per timezone. - West-bound: 0.66 day per timezone.
The asymmetry is biological: the human circadian period is slightly longer than 24 hours (~24.2 h on average), so delaying sleep (west) is closer to the natural drift than advancing (east). A jet-lag literature review (Sack 2009, NEJM) gives 1 d/tz east and 0.5–0.8 d/tz west; the calc uses 0.66 as the centerpoint.
Age penalty: max(0, (age − 25) / 10) × 0.05. Each decade beyond 25 adds 5 %. A 65-year-old has a +20 % adjustment.
Recovery_days = timezones × rate × (1 + age_penalty).
Severity bands: - 1–2 timezones: Mild. - 3–5: Moderate. - 6–8: Significant. - ≥ 9: Severe.
Trip-too-short flag: if trip_duration < recovery_days × 0.5, the user will leave before fully adjusting; staying on home time may be a better strategy than full re-entrainment for short trips.
Enter your origin UTC offset (Paris = +1, NYC = −5, Tokyo = +9). Enter the destination UTC offset. Pick the direction ("auto" reads from the offsets and signs east as positive; explicit override available). Enter the trip duration in days (so the calc can flag too-short trips). Enter your age (drives the age penalty). The result panel shows estimated recovery days, severity, time-zones crossed, direction, and a timeline with markers per day. A warning appears in accent color when the trip is too short for full adjustment.
Paris (UTC+1) → New York (UTC−5), age 35, 7-day trip.
London (UTC+0) → Tokyo (UTC+9), age 35, 14-day trip.
NYC (UTC−5) → LA (UTC−8), age 30, 5-day trip.
Average rates, individual variance. The 1.0 and 0.66 rates are population means with substantial spread. Some travellers adjust in 1 day per zone west and 1.5 east; others 2 east. The calc is a planning estimate, not a guarantee.
Light exposure dominates. Recovery accelerates with morning light exposure (entrains the eastward shift) and afternoon light avoidance for westward trips. The calc assumes default behavior; advanced jet-lag protocols (Argonne, BMJ) cut recovery time by 30–50 % with timed light exposure and melatonin.
Direction matters for east > 6 zones. Beyond 6 east-bound, the body sometimes adjusts backward (going around the long way) — a 9-zone east-bound trip can be re-entrained as a 15-zone west-bound shift, taking longer in nominal time. The calc uses the simple short-way calculation.
Age penalty is approximate. Older adults have longer phase-shift recovery, but the 5 %/decade is a smoothing of complex literature. Children and adolescents recover faster than the calc suggests (no penalty, even slightly negative).
Sleep deprivation isn't jet lag. A red-eye flight on which you didn't sleep adds 12–24 hours of acute sleep debt on top of jet lag; recovery for that part is 1–2 nights of normal sleep, separate from circadian re-entrainment.
Travel direction sign convention. The calc reads positive offset = east of UTC. Travelling east-to-west across 0 (Paris → NYC) shows |−5 − 1| = 6 zones, west-bound. Crossing the dateline (LA → Tokyo) requires care: LA is UTC−8, Tokyo UTC+9; |9 − (−8)| = 17 zones, but the short way is 7 east; the calc's auto detection takes the shorter direction.
Strict-shift vs sleep-shift. Some travellers stay partially on home time during short trips (no shift) — recovery is then 0 because there's nothing to recover. The calc assumes full re-entrainment to local time.
Effects on athletic/cognitive performance. Different abilities recover at different rates: simple reaction time within 2–3 days; complex decision-making within 5–7; immune function within a week. The calc gives one number; reality is ability-specific.
Pre-flight phase shift. Shifting your sleep schedule by 30–60 min in the right direction for 3 days before departure cuts effective recovery by 1–2 days. Apps like Timeshifter prescribe this; the calc doesn't model it.
Hidden circadian drivers. Cortisol, body temperature, melatonin all shift on different timescales. Subjective alertness can return before objective performance does — beware "feeling fine" while still impaired.