How the Jet Lag Calculator works
The calculator rests on one anchor and two rates. The anchor is your circadian temperature minimum. The rates are how fast a body clock moves in each direction. Everything the plan shows follows from those three numbers.
The anchor: your circadian temperature minimum
Your body clock has a lowest point. Core temperature bottoms out, alertness bottoms out, and the clock becomes most sensitive to light. That point is the circadian temperature minimum, written CBTmin, and it sits about 2 hours before your habitual wake time. A traveller who wakes at 07:00 has a CBTmin near 05:00.
CBTmin decides the direction light moves your clock. Light reaching your eyes after CBTmin pulls the clock earlier. Light reaching your eyes before CBTmin pushes the clock later. The same hour of sunlight therefore helps one traveller and harms another, depending on which side of their CBTmin it lands.
CBTmin moves as the plan progresses. The calculator recalculates it every day and repositions the light windows against the new position, rather than fixing them once and letting them drift out of alignment.
The direction: advance or delay
A flight east puts you in a zone whose clock is ahead of yours, so your body clock has to run earlier. That is a phase advance. A flight west puts you in a zone whose clock is behind yours, so your body clock has to run later. That is a phase delay.
The two are not equally costly. The unentrained human body clock has a natural period slightly longer than 24 hours, which means it drifts later on its own. Delaying works with that drift. Advancing works against it. Six zones east is harder than six zones west, and the calculator reflects that in both the schedule and the severity reading.
| Advance, travelling east | 1 hour per day |
|---|---|
| Delay, travelling west | 1.5 hours per day |
| CBTmin position | 2 hours before habitual wake |
| Bright light window | 4 hours, on the correct side of CBTmin |
| Low light window | 3 hours, on the wrong side of CBTmin |
| Direction reversal | Eastward shifts of 9 hours or more |
| Shift below which no plan is needed | 1 hour |
The reversal: when east becomes west
Past 9 hours east, advancing stops being the cheaper route. A traveller flying London to Auckland faces a 12 hour eastward jump. Advancing 12 hours at one hour per day takes 12 days. Delaying 12 hours at 1.5 hours per day takes eight. The clock reaches the same place either way, because a clock is a circle.
The calculator compares both routes and takes the shorter one. When it reverses direction it states the reversal in the plan summary, so you are never following an evening light instruction without knowing why it replaced a morning one.
The schedule: four phases
The plan divides into four phases. Each phase reads the clock of the place you are standing in, and the plan says which clock that is on every card.
- Prepare. Up to four days before departure, read on the origin clock. Your sleep window moves by the daily rate and the light windows follow it.
- Travel. The flight day. Set your watch to destination time at the gate. The plan marks the stretch of the flight that overlaps the night your body is moving towards.
- Arrive. The landing day, read on the destination clock. The plan states how much shift is still outstanding.
- Settle. The days after landing. The remaining shift is completed at the same daily rate, and the plan ends when your body clock matches local time.
The data: airports, zones and daylight saving
The calculator holds 3,244 airports that carry scheduled passenger service. Each one carries an IANA time zone identifier such as Europe/London or Asia/Kolkata, derived from its published coordinates. Airport and country records come from OurAirports, which releases its data into the public domain.
Time zone offsets are never hardcoded. The calculator resolves each offset for the exact instant you depart and the exact instant you land, which means a flight that crosses a daylight saving change is measured correctly. Half hour zones such as India and quarter hour zones such as Nepal and the Chatham Islands are carried through the arithmetic without rounding.
Questions about the method
Why does the calculator use the circadian temperature minimum?
The circadian temperature minimum is the single point that decides which way light moves the body clock. Light after it advances the clock. Light before it delays the clock. Any schedule built without that anchor can send a traveller in the wrong direction and make jet lag worse.
Why is the westward shift rate faster than the eastward rate?
The unentrained human body clock runs slightly longer than 24 hours, so it drifts later on its own. A westward trip asks for a later clock, which travels with that drift at about 1.5 hours per day. An eastward trip asks for an earlier clock, which travels against it at about one hour per day.
Why does a large eastward shift reverse direction?
Past nine hours east, advancing the clock costs more days than delaying it the long way round. A 12 hour eastward jump can be reached as a 12 hour delay. The calculator picks whichever direction reaches the destination clock in fewer days and says so in the plan summary.
What if I cannot start three days before the flight?
The calculator accepts zero to four preparation days. Fewer preparation days move more of the shift to the days after landing, which lengthens the plan but does not break it. The schedule always ends on the day your body clock matches local time.
Does the calculator account for the flight itself?
Yes. The travel day carries the departure time, the landing time, and the stretch of the flight that overlaps the night your body is moving towards. That stretch is marked as the sleep window to take on board.
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