The short answer

Jet lag is a mismatch between the body clock and local time after rapid travel across time zones. Lost sleep is a shortage of sleep, regardless of where the clock sits. Long-haul travel can cause both, so feeling exhausted does not by itself show how far your body clock has moved.

Jet lag is a timing problem

Your internal clock helps organise sleep, alertness and other daily rhythms. After a rapid time-zone change, that clock still expects parts of the old day. Local bedtime may arrive while your body expects to be awake.

Light is a main signal for moving that timing. The useful exposure or avoidance window depends on whether the clock needs to move earlier or later.

Travel fatigue is broader

A long trip can be exhausting without crossing several time zones. Sitting for hours, broken routines, noise, poor sleep and the work of travelling can all contribute. The 2021 athlete consensus describes travel fatigue and jet lag as distinct problems that can occur together.

Why the difference matters

Sleep can help address the need for rest. It does not automatically move the body clock toward local time. Light can help move the clock, but it does not replace sleep lost during the trip.

Moving the body clock and making up lost sleep are related jobs. They are not the same job.

A useful plan therefore needs to consider both. It should not interpret every tired period as jet lag or every sleep as circadian adjustment.

How Offset treats the two problems

Offset estimates how body time may move toward the destination and places light guidance around that estimate. Separately, it tracks the need for sleep and gives cabin or connection sleep less weight than sleep on the ground.

This is still a model. Offset does not know your exact biological phase, diagnose a sleep problem or measure how restored you feel.

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