Jet Lag Calculator

How to use the jet lag calculator

1. Fill in the fields — they start with a worked example.
2. Read the answer, and the arithmetic shown beneath it.
3. Copy the results, or adjust the inputs to compare scenarios.

About this tool

The body clock runs on a cycle slightly longer than twenty-four hours, which is why the two directions of travel are not equally hard. Flying west lengthens your day, and the clock stretches willingly. Flying east shortens it, forcing the clock to run ahead of itself, and that is the direction that costs you.

The working rule from circadian research is about one day of recovery per time zone crossed travelling west, and roughly one and a half travelling east. This calculator applies that rule and says plainly that it is a rule of thumb: individual variation is large, and age, sleep quality before the flight and how much you slept on it all move the result.

Shifting your schedule before departure is the part that actually helps, and it is the part most people skip. Moving your bedtime by an hour a day in the direction of travel for a few days before you go reduces the shift left to make on arrival. There is a limit to how much can be usefully done — beyond three or four hours it starts to disrupt the days you are still living normally.

Light is the strongest signal the body clock has, and its timing matters more than its amount. Travelling east, morning light at the destination advances the clock, which is the direction you need. Travelling west, evening light delays it. Getting this backwards can hold you where you are, which is why the advice here is specific to your direction rather than the general instruction to get some sun.

Common uses

• Estimating how many days a long-haul trip will cost you.
• Planning a schedule shift in the days before flying.
• Deciding whether to arrange meetings on the day after arrival.

Frequently asked questions

Why is flying east worse than flying west?

Because the body clock naturally runs slightly longer than twenty-four hours. Flying west lengthens the day, which suits it; flying east shortens it, which forces the clock to run ahead of itself.

How long does jet lag last?

Roughly one day per time zone travelling west and about one and a half travelling east. It is a rule of thumb, and individual variation is genuinely large.

When should I get daylight?

Travelling east, seek morning light at the destination and avoid it late. Travelling west, seek evening light. Getting the timing backwards can hold your clock where it was.