Jet Lag Calculator
A day-by-day light, sleep and caffeine plan for one specific flight — built around your temperature minimum, which is the thing everybody gets backwards.
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The one thing worth knowing: your temperature minimum
Your body temperature bottoms out roughly two hours before you habitually wake up. If you get up at seven, call it five in the morning. That moment is the hinge the whole body clock turns on, and it is the only reference point you need.
Bright light in the hours after your temperature minimum pulls your clock earlier. Bright light in the hours before it pushes your clock later. Those two sentences are the entire mechanism, and they are why generic advice to "get morning sun" is actively harmful after some flights. Fly west, and morning sun at the destination lands before your temperature minimum — it pushes you further west, away from where you are trying to get to. Fly far enough east that your temperature minimum shifts into the destination's late morning, and the same sunlight does the opposite of what you want.
This is the difference between four bad days and ten. It is also the reason this page asks what time you normally wake rather than what time you want to wake: the hinge is where your body has it, not where your calendar wants it.
East is harder, and there is a reason
Left alone in a bunker with no clocks, most people settle into a day a little longer than 24 hours. That is why staying up two hours late is easy and going to bed two hours early is nearly impossible. Flying west asks your body to do the thing it already drifts towards; flying east asks it to fight. Recovery is commonly put at about a day per time zone crossed going east and somewhat quicker going west, and that is the rate this page uses — a day per zone eastward, about a zone and a half per day westward.
Past nine or ten zones eastward the arithmetic flips. Advancing your clock by eleven hours is so hard that many travellers do better delaying by thirteen instead — going round the clock the long way. This page will say so when your flight is in that range.
What actually helps, in order
- Light, correctly timed. Nothing else is close. Everything below is a rounding error next to getting the light windows the right way round.
- Sleep timing, not sleep amount. Going to bed at the local hour matters more than the hours you log. A three-hour nap at the wrong time on day one costs you day two.
- Caffeine, as a tool and not a habit. It will hold you up through a local afternoon. It has a half-life of several hours, so a 4pm coffee is still working at midnight, and the first night is the night you cannot afford to lose.
- Shifting before you fly. An hour a day for three days is worth three days of recovery, and it is free. It is also the only item here that needs doing before you pack.
Where the shift number comes from
The shift is the difference between the destination's UTC offset when you land and the origin's when you take off — computed from the actual dates, so a flight that crosses a daylight-saving changeover gets the right answer rather than the usual one. A difference of more than twelve hours is shown as the shorter way round, because that is the direction your body clock will actually travel.
Questions people ask
Is this medical advice?
No. It is general guidance assembled from the published work on light and circadian timing, and it assumes an ordinary sleeper on no medication. If you have a diagnosed sleep disorder, take anything on a fixed schedule, are pregnant or manage a heart condition, talk to a doctor before deliberately shifting your body clock.
What about melatonin?
Deliberately left out. Melatonin shifts the clock in the opposite pattern to light, so the timing that helps is not the timing most people assume, and taking it at the wrong hour can push you the wrong way. Dose and timing both matter and both depend on you. That is a question for a pharmacist or a doctor, and in several countries it is prescription-only anyway.
Does the plan work if I cannot follow it exactly?
Yes, partly, and partial is the normal case. The windows are broad on purpose. If you only manage one thing, make it the light: get outside during the window marked "seek", and wear sunglasses during the window marked "avoid". Those two do most of the work.
Why does it ask when I normally wake rather than when I want to wake?
Because the temperature minimum — the point the whole plan is measured from — sits about two hours before your habitual wake time, not your intended one. Enter 6am when you actually get up at 8am and every light window on the page moves two hours in the wrong direction.
Is my flight information sent anywhere?
No. Everything is computed in your browser and nothing is stored, logged or uploaded. Close the tab and it is gone.
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Pages like this one get linked to because somebody needed the answer and found it here. That is the same reason anything gets linked to, and it is the part most link building skips.