How jet lag works, and how this plan is made

Your body keeps its own time. A clock in the brain sets when you feel sleepy, when you are alert, when you are hungry and when your temperature dips to its lowest point, which for most people is about three hours before they normally wake. Fly across several time zones and that clock is still set to home. Jet lag is the days it takes to catch up.

The clock is moved mainly by light, and the direction depends on when the light arrives. Light in the hours before your body’s low point pushes the clock later. Light in the hours after it pulls the clock earlier. That is why the same morning sunshine can help one traveller and set another back: it depends on what time their body thinks it is.

What the planner does

It places your body’s low point from your usual wake time, works out how far the clock has to move, and moves it by about one hour a day when you fly east and about an hour and a half a day when you fly west, which are the rates sleep researchers generally work with. For each day it then marks the hours when light helps and the hours when light would work against you, and suggests a sleep window that moves towards local night without asking you to sleep when your body cannot.

Why east is harder than west

Most people’s natural day runs a little longer than 24 hours, so staying up late is easier than falling asleep early. Flying west asks you to stay up; flying east asks you to sleep before you are tired. For very long eastward trips of nine or ten time zones or more it is often quicker to shift the clock later, the long way round, and the planner chooses that when it is faster.

Common questions

How long does jet lag last? A common rule is one day for each time zone crossed going east and a day for every zone and a half going west. Well-timed light can keep you at that pace; light at the wrong time can add days.

Should I start before I leave? If you can move your bedtime and waking time for two or three days before the flight, you land with part of the work already done. The planner shows those days when you choose to start early.

What counts as bright light? Daylight outdoors, even under cloud, is many times brighter than indoor lighting. To seek light, go outside. To avoid it, stay indoors with the lights low, or wear dark glasses if you must go out.

Does melatonin help? Studies support a small dose taken at the right time when travelling east. The timing matters more than the amount. It is sold freely in some countries and needs a prescription in others, and it is not suitable for everyone, so ask a doctor or pharmacist first.

What about a short trip? If you are away for two or three days, it is usually better not to shift at all. Keep your home sleep hours as far as the trip allows.

Where do the flight times come from? The travel time is an estimate from the great-circle distance between the two airports at a typical cruising speed. Replace it with the time on your ticket, including any stopover.