Jet Lag Planner

How to Get Daylight When It's Cloudy or Dark: Weather's No Excuse

You wake to a flat grey sky, look out the window, and decide there's no point going out for light today. It's the easiest call in the world to make, and it's wrong nearly every time, because it's the instinct that wrecks a jet-lag plan. A cloudy day still pours somewhere between 1,000 and 25,000 lux into your eyes, against the 200 to 500 you get from even a well-lit office. So on a grim day the first move is the same as on a glorious one: get outside anyway. The one real exception is when there is no daylight to be had at all, before dawn, deep in a polar winter, or stuck somewhere you simply cannot get out, and that is when a 10,000-lux light box finally earns its keep.

A grey sky is far brighter than it looks

Your eyes are so good at adjusting that a cloudy afternoon and a sunlit office can honestly feel about the same. A light meter is not fooled for a second. A dull overcast sky reads around 1,000 lux, and a bright cloudy one climbs well past 10,000, so cloudy daylight runs from a couple of times brighter than a decent indoor room at the gloomy end to twenty times or more at the bright end. Your clock cares about that raw number, not about how bright the place happens to look to you, and that is the crux of the whole thing. What it means in practice is that on a grey day your outdoor light is weaker than in full sun, so you give it a bit longer; it's still doing the job. So does daylight really work on a cloudy day? It does, comfortably.

Rain is not a day off

If your plan says chase light, treat rain and cloud as a reason to grab a jacket rather than an excuse to stay in. Take your morning coffee out on the step, or eat breakfast propped in an open doorway when you can't face the drizzle properly. A walk of 30 to 60 minutes inside your seek-light window does it, and a dull day earns the longer end. Keep the sunglasses off while you're chasing light and save them for the windows where you're meant to be dodging it, and don't fret about finding the sun itself; just face the brightest patch of sky and let it count.

A window is a poor stand-in for the outdoors

Sitting by a window feels like getting daylight, and mostly you aren't. Glass strips out part of the light, and the intensity drops off fast as you back away, so a seat a metre from a bright window might hand you only a few hundred lux, office territory again. Pressed right up against a big, bright pane is better. None of it touches actually being outside. Use a window when you can't step out, and go in knowing you are throwing most of the dose away.

When a light box actually makes sense

A 10,000-lux light box, the kind people lean on through dark northern winters, is a fair stand-in for the times daylight simply isn't on offer: a seek-light window that opens before sunrise, a high-latitude winter with barely any daytime, a long layover in a windowless terminal, any morning you physically can't get out. You don't have to own one, though. For most trips, walking outside is cheaper and brighter, and the planner treats a box as an option for the dark hours rather than something you're expected to buy.

If you do use one, this is roughly how the clinics that study light therapy go about it. You want about 30 minutes at 10,000 lux, sitting somewhere around 40 to 60 cm (call it 16 to 24 inches) from your face, with the light angled so it reaches your eyes from the side instead of glaring straight into them; you don't stare at it. And you time it exactly as you would daylight, by your low point: use it in your seek-light window, which is your morning if you are advancing for an eastward trip, and skip it in the avoid-light windows, where it would only push you the wrong way. If that low-point business is new, Morning Light or Evening Light: Which Way You Shift Your Clock explains which way to aim it, and Light: The Most Powerful Jet-Lag Lever covers why light beats every other trick in the first place.

When "just go outside" breaks down

Two situations beat the go-outside rule, and both come down to there being no sun to go out into. The first is a light window that opens before the sun does. If your plan wants bright light at 5:30 local and the sky is still black, a box bridges the gap until real daylight shows up, and then you switch to being outside. The second is deep-winter darkness at high latitude, where you might get a few hours of thin daylight or none at all, so you grab whatever natural light lands around midday and fill the rest of your seek-light window with the box.

The polar summer flips the same problem on its head, which catches people out. When the sun barely sets, it is the avoid-light windows that turn into the hard part, and blackout curtains, a sleep mask, and sunglasses late in the evening are suddenly what protect your shift. It's a useful reminder that this is all about controlling light in both directions, not only chasing it, and that first day or two is where getting it right pays off most, which is the whole point of Your First 48 Hours at the Destination.

Fun fact

Your eyes can cope with a genuinely staggering range of brightness, from a moonless night at well under a hundredth of a lux to the glare of high noon at a hundred thousand: a span of something like a billion to one. They manage it by constantly rescaling what "bright" means, which is the exact reason a cloudy day and a lit office can feel identical to you. Your body clock never got that upgrade. It just counts the light actually landing on it, and outdoors that count is always, always higher.

How the Jet Lag Planner handles this

The Jet Lag Planner gives you the clock time of every seek-light and avoid-light window, so you know exactly when a soggy walk or a light-box session should happen and when to keep things dark instead. It won't sell you a device or assume you already own one; a light box turns up only for the hours with no daylight at all. Enter your route and it shows you your own windows.

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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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.