Jet Lag Calculator
Estimate how many days you need to recover from jet lag and get a day-by-day light, sleep, and caffeine plan for eastward or westward travel.
Difference in hours between home and destination (e.g. New York → London = 5).
Before you fly (3 days of pre-shifting)
- Day −3 (before flight)Go to bed and wake up 1 h earlier than the day before (1 h earlier than your usual schedule by tonight). Get bright light in the morning; keep the evening dim.
- Day −2 (before flight)Go to bed and wake up 1 h earlier than the day before (2 h earlier than your usual schedule by tonight). Get bright light in the morning; keep the evening dim.
- Day −1 (before flight)Go to bed and wake up 1 h earlier than the day before (3 h earlier than your usual schedule by tonight). Get bright light in the morning; keep the evening dim.
After you land — day by day
- Day 1 after landingBody clock still ≈ 5 h behind local time. Get 20–30 min of outdoor light soon after waking, and dim the lights an hour before local bedtime. Keep meals on the local schedule.
- Day 2 after landingBody clock still ≈ 4 h behind local time. Get 20–30 min of outdoor light soon after waking, and dim the lights an hour before local bedtime. Keep meals on the local schedule.
- Day 3 after landingBody clock still ≈ 3 h behind local time. Get 20–30 min of outdoor light soon after waking, and dim the lights an hour before local bedtime. Keep meals on the local schedule.
- Day 4 after landingBody clock still ≈ 2 h behind local time. Get 20–30 min of outdoor light soon after waking, and dim the lights an hour before local bedtime. Keep meals on the local schedule.
- Day 5 after landingBody clock still ≈ 1 h behind local time. Get 20–30 min of outdoor light soon after waking, and dim the lights an hour before local bedtime. Keep meals on the local schedule.
- Day 6 after landingBody clock roughly aligned with local time. Get 20–30 min of outdoor light soon after waking, and dim the lights an hour before local bedtime. Keep meals on the local schedule.
Every day of the plan
- ☀️Seek bright morning light; keep evenings dim.
- ☕Last caffeine at least 8 hours before your target local bedtime — check the caffeine half-life calculator to see what is still in your system.
- 😴If you must nap, keep it under 30 minutes and before mid-afternoon local time.
- 🍽️Eat meals on the destination's schedule from day one.
How the estimate works
Jet lag happens because your internal circadian clock is still running on your departure city's time. The clock re-synchronizes on its own, but slowly: travel-medicine guidance such as the CDC Yellow Book and the American Academy of Sleep Medicine describe a widely used rule of thumb — the body clock shifts about 1 hour per day after flying east and about 1.5 hours per day after flying west. That is the whole formula behind this calculator: eastward recovery ≈ one day per time zone crossed, westward recovery ≈ one day per 1.5 zones, rounded up.
Eastward travel is harder because it demands a phase advance — falling asleep earlier than your body wants — while the human clock naturally runs a little longer than 24 hours and prefers to delay. That is also why the plan differs by direction: after flying east you chase morning light to pull your clock earlier; after flying west you chase late-afternoon and evening light to push it later. Starting 2–3 days before departure, shifting your bedtime and wake time by one hour per day in the direction of travel takes a real bite out of the adjustment you have left to do on arrival.
The supporting habits matter as much as the light: cut caffeine at least 8 hours before your target local bedtime (our caffeine half-life calculator shows why a 4 p.m. coffee still has plenty of kick at midnight), keep naps under 30 minutes and before mid-afternoon, and eat meals on the local schedule from day one. None of this is medical treatment — it is schedule management, and it works best done consistently for every day of the adjustment window.
Consistency is the hard part, especially while traveling. A simple checklist habit — morning light walk, caffeine cutoff, no late naps — tracked in HabitBox for the week around your trip turns the plan into three one-tap check-ins a day instead of something you have to remember while jet-lagged.
Note: this tool gives a general schedule estimate for healthy adults. It is not medical advice — if you have a sleep disorder, take medication on a strict schedule, or fly this route constantly for work, talk to a clinician or travel-medicine specialist.
Frequently asked questions
How long does jet lag last?+
The common rule of thumb in travel-medicine guidance (CDC Yellow Book, American Academy of Sleep Medicine) is that the body clock adjusts about 1 hour per day after flying east and about 1.5 hours per day after flying west. So a 6-zone eastward trip takes roughly 6 days to fully adjust, while the same trip westward takes about 4. Most people feel noticeably better before they are fully adjusted.
Why is eastward jet lag worse than westward?+
Flying east means going to bed earlier than your body clock wants — a phase advance — and the human circadian rhythm, which naturally runs slightly longer than 24 hours, finds it easier to delay (stay up later) than to advance. That is why the rule of thumb allows about 1.5 zones per day of recovery westward but only about 1 zone per day eastward.
How does light exposure help with jet lag?+
Light is the strongest signal that sets the circadian clock. After flying east, bright morning light helps pull your clock earlier; after flying west, bright late-afternoon and evening light helps push it later. Getting light at the wrong time (for example, very early morning light after a large eastward shift) can temporarily push the clock the wrong way, which is why the plan times light exposure by direction.
Should I nap after a long flight?+
Short naps are fine and can take the edge off — keep them under about 30 minutes and before mid-afternoon local time. Long or late naps reduce sleep pressure and make it harder to fall asleep at the local bedtime, which slows down your adjustment.
Is it worth adjusting for short trips?+
If you cross only 1–2 time zones, or you are staying fewer than about 3 days, many travel-medicine sources suggest simply keeping your home schedule where practical rather than forcing a full shift. For longer trips across 3+ zones, pre-shifting your sleep by 1 hour per day for 2–3 days before departure gives you a head start.
Make the adjustment plan stick
The plan only works if you actually do it every day of the trip. HabitBox is a free, ad-free habit tracker — add 'morning light walk', 'caffeine cutoff', and 'no late naps' for your travel week and check them off in one tap. No account, data stays on your device, iOS and Android.
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