Jetlag Away

The core body temperature minimum: the hour that decides which way light moves you

The core body temperature minimum is the lowest point of your body’s daily temperature rhythm, and it falls in the second half of your usual night. It matters for jet lag because it is the hinge of the light response: light in the hours before it moves your body clock later, light in the hours after it moves the clock earlier. It cannot be felt, but it can be estimated from when you normally sleep.

Light timing explains what to do with light. This page is about the marker the light instructions are built around: what it is, where it sits, how an app can estimate it, and how wrong that estimate is allowed to be.

A low point you sleep through

Core body temperature is not constant. It rises through the day and falls through the night, and the bottom of that curve, the core body temperature minimum (often written CBTmin or Tmin), arrives a few hours before you would naturally wake. For most people it passes unnoticed, because they are asleep.

On its own that would be trivia. What makes it useful is that the temperature rhythm is driven by the same internal clock that decides when you feel sleepy and when you feel alert. Where the minimum sits is a readout of where your body clock is.

Why light flips direction at that hour

The body clock’s response to light has two halves, and the minimum is where they meet:

In the study that measured this most directly, the change from one effect to the other was rapid and sat at the minimum itself, and there was no neutral stretch where light did nothing [KJ03]. The largest shifts come from light in the three to six hours either side of it [RS19].

That is why “get some sunlight” is incomplete advice for a traveller. Whether that sunlight helps depends on which side of the minimum it lands, and after a long flight your minimum is not where the local clock suggests.

How it is estimated

Measuring the real minimum needs a laboratory: a core temperature sensor, hours of controlled conditions, or repeated samples of melatonin in dim light. No app does that. What an app can do is estimate it from your habitual sleep, which the research shows is a reasonable guide:

Jetlag Away uses your usual bedtime plus five hours, which lands inside both. If you normally sleep from 23:30 to 07:00, your estimated minimum is 04:30. A later sleeper gets a later estimate, which is why the app asks for your real hours rather than assuming an average traveller’s.

How wrong the estimate is allowed to be

It is an estimate, and the plan treats it as one. The Eastman and Burgess range alone is an hour and a half wide, and individuals differ. So the light windows are placed with a margin:

Where it goes when you fly

Your minimum does not travel at the speed of the plane. On the first morning after landing it is still where your home clock left it, less whatever the nights before departure managed to move it.

Take a Copenhagen traveller with a 23:30 bedtime flying to Tokyo in summer, seven hours east. With nothing shifted beforehand, the estimate is still 04:30 Copenhagen time, which is 11:30 in Tokyo. On that first morning, a walk in the sun at nine o’clock lands before the minimum: a strong dose of the delaying signal on a trip that needs the advancing one. The plan’s instructions for that day follow from the estimate: keep out of bright light until the low point is safely behind you, which on that first day runs into the early afternoon, and seek it once the window on the advancing side opens.

Each day after that, the plan moves the estimate by the day’s assumed rate towards your normal 04:30 on the local clock, and the light windows move with it. That stepping is what turns a fixed rule into a schedule.

Before you fly, it moves too

The nights before departure are the cheapest place to move the minimum, because at home you control your light. When the plan moves your bedtime earlier or later on those nights, the estimate moves with it, and each moved night is paired with a light window on the correct side: the morning after an earlier night, or the evening before a later one. In one study, shifting sleep an hour earlier each day moved the clock about 0.6 hours in dim light, and 1.5 to 2.1 hours with bright morning light [BC03]. The minimum is what decides which morning hour counts as “after”.

What the app tells you

You never have to calculate any of this. Each light instruction in the plan names the hours and the estimate behind them, so “avoid bright light until 11:30” arrives with its reason, and the Circadian Trajectory screen shows the estimated shift day by day, labelled as an estimate from the plan’s own model rather than a measurement. How a plan is built has the full set of rules the windows follow.

Sources

Jetlag Away is a scheduling tool and not medical advice. If you have a sleep or circadian disorder, or any diagnosed condition affected by sleep, talk to a healthcare professional first — see the health disclaimer in our terms.

Last reviewed 10 October 2026. Published 10 October 2026.

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