Jet Lag Calculator
How long jet lag lasts for your flight, why flying east hurts more, and a day-by-day light-exposure plan built on your body clock's temperature minimum.
Jet Lag Calculator: with the default inputs, days to feel normal is 6.
New York to London is 5; New York to Paris 6; Los Angeles to Tokyo 16 hours ahead, which is 8 zones the short way round.
Eastward means the destination clock is ahead of home.
24-hour clock. Your body's temperature minimum — the hinge of the whole plan — sits about 2 hours before this.
Short trips are often best handled by staying on home time.
- Expected severity
- Significant — 6 zones east
- What your clock has to do
- Advance (move earlier) by 6 hours
- Realistic shift per day
- 1.00 hours per day
- Body-clock low on arrival day
- 11:00 AM destination timeYour temperature minimum, expressed in destination local time. Light before it delays your clock; light after it advances it.
- Seek bright light (arrival day)
- 11:00 AM – 3:00 PM
- Avoid bright light (arrival day)
- 7:00 AM – 11:00 AM
- Worth adjusting?
- You have 10 days, enough to adjust fully in about 6. Shift deliberately from day one — a fixed local wake time and the light windows below are worth more than anything you can buy at the airport.
Assumptions
- Re-entrainment is modelled as a constant rate: 1.0 h/day advancing, 1.5 h/day delaying, both editable.
- The core body temperature minimum is taken as 2 hours before habitual wake time.
- Eastward shifts of more than 8 time zones are planned as delays the long way round.
- Melatonin, naps, caffeine and in-flight sleep are not modelled.
| Clock still off by (h) | Seek bright light | Avoid bright light | Aim to be asleep | Aim to wake | |
|---|---|---|---|---|---|
| Arrival day | 6 | 11:00 AM – 3:00 PM | 7:00 AM – 11:00 AM | 5:00 AM | 1:00 PM |
| Day 2 | 5 | 10:00 AM – 2:00 PM | 6:00 AM – 10:00 AM | 4:00 AM | 12:00 PM |
| Day 3 | 4 | 9:00 AM – 1:00 PM | 5:00 AM – 9:00 AM | 3:00 AM | 11:00 AM |
| Day 4 | 3 | 8:00 AM – 12:00 PM | 4:00 AM – 8:00 AM | 2:00 AM | 10:00 AM |
| Day 5 | 2 | 7:00 AM – 11:00 AM | 3:00 AM – 7:00 AM | 1:00 AM | 9:00 AM |
| Day 6 | 1 | 6:00 AM – 10:00 AM | 2:00 AM – 6:00 AM | 12:00 AM | 8:00 AM |
| Day 7 | 0 | 5:00 AM – 9:00 AM | 1:00 AM – 5:00 AM | 11:00 PM | 7:00 AM |
Advancing: light in the hours after your temperature minimum pulls the clock earlier; light in the hours before it pushes the clock the wrong way. Sunglasses and dim rooms are the 'avoid' tool.
How this is worked out
The formula
clock shift needed = time zones crossed (eastward beyond 8 zones: 24 − zones, taken as a delay) days to adjust = clock shift ÷ rate, where rate ≈ 1.0 h/day advancing and ≈ 1.5 h/day delaying temperature minimum (home clock) ≈ habitual wake time − 2 h temperature minimum (destination clock, arrival) = home minimum ± time zones crossed advance: seek light after the minimum, avoid it before delay: seek light before the minimum, avoid it after
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- Time zones crossed
- New York to London is 5; New York to Paris 6; Los Angeles to Tokyo 16 hours ahead, which is 8 zones the short way round.from 0 to 14 · whole numbers only · defaults to 6
- Direction of travel
- Eastward means the destination clock is ahead of home.Eastward (clocks go forward — you lose hours) · Westward (clocks go back — you gain hours)
- Usual wake time at home
- 24-hour clock. Your body's temperature minimum — the hinge of the whole plan — sits about 2 hours before this.from 0 to 23 · whole numbers only · defaults to 7
- Sleep you normally get
- A number.from 4 to 12 · defaults to 8
- Length of stay
- Short trips are often best handled by staying on home time.from 1 to 365 · whole numbers only · defaults to 10
- Adjustment rate flying east(under More options)
- The clock advances roughly 1 hour a day unaided.from 0.25 to 3 · defaults to 1
- Adjustment rate flying west(under More options)
- Delays come easier: roughly 1.5 hours a day, because the free-running human clock is slightly longer than 24 hours.from 0.25 to 3 · defaults to 1.5
- Length of the light window(under More options)
- How long a block of bright light to seek or avoid either side of the temperature minimum.from 1 to 8 · defaults to 4
What you get back
- Days to feel normalmain answer
- Expected severity
- What your clock has to do
- Realistic shift per day
- Body-clock low on arrival day
- Your temperature minimum, expressed in destination local time. Light before it delays your clock; light after it advances it.
- Seek bright light (arrival day)
- Avoid bright light (arrival day)
- Worth adjusting?
What this assumes
- Re-entrainment is modelled as a constant rate: 1.0 h/day advancing, 1.5 h/day delaying, both editable.
- The core body temperature minimum is taken as 2 hours before habitual wake time.
- Eastward shifts of more than 8 time zones are planned as delays the long way round.
- Melatonin, naps, caffeine and in-flight sleep are not modelled.
About this calculator
Jet lag is not tiredness. It is a mismatch between the clock on the wall and the clock in your hypothalamus, and it takes days to resolve because that internal clock only moves about an hour a day. This calculator estimates how many days you need, tells you which way your clock has to move, and gives you the light windows that actually move it.
Why east is worse
The unaided human circadian period averages about 24.2 hours — slightly longer than a day. Left alone in constant conditions, people drift later. That makes delaying (flying west, staying up later) the direction your body already wants to go, and it re-entrains at roughly 1.5 hours a day. Advancing (flying east, going to bed earlier) fights the drift and manages about 1 hour a day. Six zones west is about four days; six zones east is about six. Beyond eight zones east, the shift is often easier the long way round as a delay, and the calculator switches to a delay plan and says so.
The temperature minimum is the hinge
Light is the signal that moves the clock, but its effect flips sign around your core body temperature minimum, which falls roughly two hours before your habitual wake time. Light in the hours after the minimum advances your clock. Light in the hours before it delays your clock. Get this backwards and you will push yourself further from local time while diligently "getting sunlight".
So the plan the table lays out is: find where your temperature minimum lands on the destination clock, then chase or dodge light on the correct side of it, moving the target each day as the clock shifts.
How to use it
Enter the zones crossed and the direction, plus your usual wake time — that one number sets the whole schedule. Length of stay decides whether adapting is worth it at all: if the trip is shorter than the recovery estimate, staying near home time is often the better trade, and the calculator says so. Under More options you can change the assumed adjustment rates and the length of the light block.
What it will not do
These are population averages. Individual re-entrainment varies a lot, older travellers tend to adapt more slowly to eastward flights, and evening types generally find westward travel easier. Melatonin (0.5–3 mg, timed relative to the same temperature minimum) and short strategic naps help some people; the calculator does not model them. Nothing here is medical advice, and if you fly across zones constantly for work, the effects on sleep debt are worth discussing with a doctor rather than out-engineering.
Frequently asked questions
▸How long does jet lag last?
Roughly one day per time zone when flying east and one day per one-and-a-half zones flying west. Six zones east is about six days; six west is about four. Well-timed light can roughly halve that.
▸Why is flying east worse than flying west?
The human circadian clock free-runs at about 24.2 hours, so it drifts later on its own. Flying west asks it to do what it already wants; flying east asks it to fight that drift, and it moves about a third slower.
▸When should I get sunlight after a flight?
It depends which way you flew. Flying east, get light in the hours after your body-clock low (about two hours before your usual home wake time, read on the destination clock) and avoid light before it. Flying west, do the opposite.
▸Should I bother adjusting for a two-day trip?
Usually not. If the trip is shorter than the recovery estimate, keeping close to home time — scheduling what matters during your home-time day — costs less than adapting twice in a week.
▸Does jet lag get worse with age?
Eastward adaptation tends to slow with age, and older travellers report more disrupted sleep after time-zone change. Westward travel shows much less of an age effect.
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