How jet lag works

Jet lag is not tiredness. You can be jet-lagged and wide awake — at three in the morning, in a hotel room, having slept eight hours on the plane. What has gone wrong is not the amount of sleep but its timing, and the timing is set by a clock that did not travel with you at the same speed you did.

There is a clock, and it is physical

Almost every tissue in the body runs on a daily cycle, and those cycles are kept in step by a small region of the hypothalamus called the suprachiasmatic nucleus. It governs when core body temperature falls, when melatonin is released, when alertness peaks and troughs. Left entirely alone — no daylight, no clocks, no schedule — that rhythm does not run to exactly twenty-four hours. For most people it runs slightly long.

That detail sounds like trivia and it is the reason jet lag behaves the way it does. A clock that naturally drifts later is a clock that finds staying up later easy and going to bed earlier hard. Every asymmetry described on this site follows from it, including why flying east is harder than flying west.

Because the internal period is not exactly twenty-four hours, it has to be corrected daily against the outside world. The signals that do the correcting are called zeitgebers — “time-givers”. Light is overwhelmingly the strongest of them.

Flying moves the destination, not the clock

When you fly from Copenhagen to Tokyo, the local time changes by seven or eight hours the moment you land. Your circadian system does not do anything of the kind. It carries on running on Copenhagen time, and it will keep running on Copenhagen time until something persuades it otherwise.

So for the first days after arrival, two clocks disagree. The one on the wall says it is nine in the morning and the city expects you to be working. The one in your hypothalamus says it is two at night and has scheduled your lowest body temperature, your lowest alertness, and the tail end of melatonin release. The symptoms people describe as jet lag — waking at four and being unable to get back to sleep, an afternoon that feels like the middle of the night, digestion that has opinions about all of this — are what that disagreement feels like from the inside.

This is also why “just push through and go to bed at the normal local time” is unreliable advice. Willpower controls when you lie down. It does not control when melatonin is released.

Adjustment is slow, and roughly linear

The clock does move. Unaided, it shifts on the order of an hour a day when it has to move earlier and an hour and a half when it has to move later. Those are the figures Jetlag Away’s gentle setting is built on; the faster settings assume that correctly timed light does better, by amounts the plan page states. Either way the length of a plan scales with the size of the shift rather than being a fixed number of days.

A four-hour shift is therefore a few days of adjustment. A nine-hour shift, left to itself, is closer to a week — which is longer than a great many trips. The practical consequence is uncomfortable: on a short trip you can easily fly home before your body clock has finished arriving, and then adjust back. That is a real scenario, not a failure to try hard enough, and it is one of the cases where deliberately not adjusting is the better plan.

What changes the arithmetic is deliberate control of the signals the clock listens to. Timed correctly, light can move the clock substantially faster than it drifts on its own. Timed carelessly, it can move it the wrong way, which is the failure mode most people never find out about because nothing announces it. That is the subject of light timing.

Why the number of hours is not the whole story

Two facts about a flight decide most of what the adjustment will be like: how many hours the clock has to move, and in which direction. Everything else — the length of the flight, the airline, whether you slept on board — matters much less than those two.

A shift is also not the same as a flight duration. Copenhagen to Tokyo is a long flight and a large shift. Copenhagen to Johannesburg is a comparably long flight and almost no shift at all, because it is a journey north to south. You can spend eleven hours in a seat and land with no circadian adjustment to make whatsoever. You will be exhausted, but you will not be jet-lagged, and the two need different remedies: one needs a night’s sleep, the other needs several days of correctly timed light.

What this means in practice

The useful conclusions from all of the above are unglamorous:

• The adjustment starts before you fly, or it starts late. Moving your schedule even an hour toward the destination in the days beforehand is an hour you do not have to make up after landing.
• Direction matters more than distance. Check which way the clock has to move before deciding anything else.
• Timing beats effort. Light at the right hour does more than any amount of determination about bedtime.
• A short trip may not be worth adjusting for at all.

Jetlag Away exists to work out the specifics — which hours, which direction, on which days — from your actual flight. How a plan is built describes what it does with them.

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.