Jet Lag Planner

Light and Jet Lag: Your Most Powerful Lever (and It's Free)

The best jet-lag fix you have is free, and it's the kind of thing you walk straight past on the way to the taxi rank. Of everything you can do about jet lag, catching bright light at the right time moves your body clock harder than caffeine, a decent meal, a gym session, or any sleep aid you've reached for. Light is the master signal your brain reads to work out what time it is, which means the highest-value thing you can do after landing is almost insultingly basic: go outside and stand in it. Open daylight runs something like 20 to 200 times brighter than the light indoors, and even a filthy overcast sky is still a couple of times brighter. That gap is the whole reason it works.

Why light wins

Deep in your eyes sit a handful of cells whose only job is to report how bright the world is. They wire straight into your body's master clock, a tiny knot of tissue buried in the brain (the suprachiasmatic nucleus, if you ever want to win a pub quiz). No other sense gets a private line like that. Researchers call light the dominant zeitgeber, German for "time-giver," and the scientists behind the most-used travel protocols put it at the top of every plan they build.

Food, exercise and a normal social schedule do help, mostly by making a new routine stick once you're on the ground. As things that physically haul your clock across time zones, though, they're featherweights next to daylight. When one lab ran the same protocol on both, an hour of exercise moved the clock about a third as far as three hours of bright light did, which is exactly why this site rates light and caffeine timing as strong advice and meal timing as weaker. Left to sort itself out, your clock only shuffles across an hour a day heading east and an hour and a half heading west. Aim your light on purpose and those become roughly an hour and a half and two hours, which is how you get ahead of the crawl instead of waiting a week for it.

You might wonder whether light really beats melatonin, and for the specific job of moving your clock it's the stronger and better-studied tool, plus it costs nothing and no country requires a prescription for going outdoors. Melatonin's rules are messier: prescription-only in the UK, a shelf supplement in the US, and a patchwork across the EU, Australia and New Zealand. It does have real trial evidence behind it, and whether it earns a place in your trip is a conversation for you and your doctor, which is why we've built the plans here on light instead. If you want the melatonin side of the argument, Melatonin and Jet Lag: What the Science Says lays it out.

The brightness gap

Lux is just a way of measuring how much light actually lands in your eyes, and when you line the numbers up they turn out wildly lopsided:

Where you are Light hitting your eyes
Direct summer sun 32,000-100,000 lux
Full daylight, no direct sun 10,000-25,000 lux
Bright but cloudy day a few thousand up to 10,000 lux
Dull, heavy overcast around 1,000-2,000 lux
Very dark overcast day around 100 lux
Typical office or shop 200-500 lux
Living room in the evening 50-150 lux

A well-lit office feels bright because your eyes are extraordinary at adjusting, but your clock is not fooled for a second. It reads raw intensity, and at 200 to 500 lux, indoor light is a whisper. Step out into open daylight and you hand it twenty to two hundred times more. Step out under heavy cloud and it's only two to five times more, which is the honest number and still the right side of the trade. That, in a single line, is why "go outside" beats nearly every other trick going.

Your screen won't do it, and a window barely will

It's tempting to treat an hour on your laptop by the hotel window as basically daylight. It isn't, not remotely. A screen throws maybe a few tens of lux at your face, a rounding error next to the sky, and it only really matters for jet lag in one direction: late at night, it's a dose of light that can shove your clock later when you are trying to wind down. A window is better but still thin gruel, because glass eats part of the light and the dose falls off fast as you back away. Sit a metre inside a bright window and you might collect a few hundred lux, office territory, when the open air a few steps beyond it would give you fifty times that.

Cloud is not the excuse it looks like

A grey sky feels like the sun has knocked off for the day. It hasn't. A dull, heavily overcast day still measures around 1,000 to 2,000 lux, and a bright cloudy one runs to 10,000 or more, so even the gloomy end is several times a lit room and the bright end leaves it in the dust. If your plan says get light, the weather is almost never a good enough reason to stay in; you just give it a little longer. How much longer? There's no magic number, because it rides on how bright it is and how well you've timed it, but a solid block of very roughly 30 to 60 minutes inside your seek-light window does the job, and the full hour is the safer bet on a properly miserable day. A morning walk or a coffee on a bench counts.

When there is genuinely no daylight to be had, a pre-dawn start or the black middle of a northern winter, a 10,000-lux therapy lamp is a reasonable backup, used up close. Even then it only lights a small cone at arm's length, where the open sky throws as much or more across everything you can see, for free, the second you step out. If you are wrestling with grim weather or darkness a lot of the time, How to Get Enough Daylight When It's Cloudy or Dark gets into the workarounds properly.

The catch is timing

Here's the part that trips people up, and it's why "get light" is only ever half an instruction. Light is strong in both directions, which makes it as easy to hurt yourself as to help yourself. The same bright morning that rescues you after flying east can flatten you after flying west. Your clock swings on a hinge: the coldest point of your night, which the clinical protocols put about 3 hours before you'd normally wake and the CDC puts 2 to 4 hours before. It's the star of Your Body Clock: Circadian Rhythm and the Temperature Minimum if you want the mechanism. Catch light in the four hours after that point and you shift earlier; catch it in the four hours before, and you shift later. Aim it at the wrong hour and it will happily drag you the wrong way. Knowing when to chase light and when to hide from it is the rest of the instruction, and it's the whole of the next article, Morning Light or Evening Light: Which Way You Shift Your Clock.

Fun fact

For more than a century everyone agreed the eye had exactly two kinds of light detector: rods and cones, the cells you actually see with. Then, around the year 2000, scientists found a third. A sparse scatter of cells that barely help you see at all, wired almost entirely to your body clock and running on a light-catching pigment called melanopsin. You have been carrying a dedicated "what time is it?" sensor in each eye your whole life, and nobody noticed until the turn of the millennium.

How the Jet Lag Planner handles this

The Jet Lag Planner is built around light because light does the heavy lifting; you punch in your route and dates and it plays your trip back as a moving map sliding through day and night, then hands you plain seek-light and avoid-light windows in your own local time, day by day. It won't promise to cure your jet lag and it stays well clear of prescription territory, but it will point you at the one lever with the best evidence behind it and tell you the hours to pull it.

Sources

General wellness information, not medical advice. Talk to a clinician before changing your schedule if you have a sleep or health condition.

Last reviewed August 5, 2026.

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Jet Lag Planner

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General wellness information, not medical advice. Talk to a clinician before changing your schedule if you have a sleep or health condition. © 2026 XOR Limited.