Why light matters more than anything else
If you only do one thing to manage jet lag, get your light exposure right. Light is the most powerful signal your body uses to set its internal clock. It's not a nice-to-have. It's the primary mechanism.
Here's how it works. Specialised cells in your eyes (called intrinsically photosensitive retinal ganglion cells) detect light and send signals directly to your brain's master clock, the suprachiasmatic nucleus. This clock controls your sleep-wake cycle, hormone release, body temperature, and dozens of other rhythms. When the light signals match the local day-night cycle, your body is synchronised. When they don't, because you've flown across time zones, your body is in jet lag.
No supplement, diet, or hack is as effective as timed light exposure for resetting this clock. Melatonin helps, but it's a supporting act. Light does the heavy lifting.
Timing determines direction
This is where most advice goes wrong. People say "get lots of sunlight," but the timing of that light determines whether your clock shifts forward (earlier) or backward (later). Get it wrong and you'll make your jet lag worse.
Light in the morning (before noon) shifts your clock forward: it makes you want to sleep and wake earlier. This is what you want after eastbound travel.
Light in the evening (afternoon to sunset) shifts your clock backward: it makes you want to sleep and wake later. This is what you want after westbound travel.
Light at the wrong time can shift your clock in the opposite direction. For example, if you've flown east and you get bright light in the evening at your destination, you're pushing your clock later, exactly the wrong way.
Practical guidelines by direction
After flying east (UK to Asia, US to Europe, and so on): seek bright light between 6am and 11am at your destination. Get outside, because even overcast daylight is far brighter than indoor lighting. Avoid bright light in the evening. Wear sunglasses if you're out after 6pm for the first couple of days. If you wake in the middle of the night, keep the lights dim. Don't check your phone (the screen light tells your brain it's daytime).
After flying west (UK to US, Asia to Europe, and so on): seek bright light in the afternoon and early evening at your destination. Stay outside or in bright environments until 7 or 8pm. Morning light is fine but less important on this route. The natural tendency to stay up late works in your favour here, so don't fight it too hard on the first night.
How bright is bright enough?
Natural outdoor light is far more powerful than indoor lighting, even on a cloudy day.
A sunny day delivers roughly 10,000 to 100,000 lux. An overcast day still delivers 1,000 to 10,000 lux. A brightly lit office delivers about 300 to 500 lux. Your living room at night is typically 50 to 150 lux.
For circadian resetting, you need at least 1,000 to 2,000 lux, ideally more. That's why "get outside" is the advice, not "turn on more lamps." Even 20 to 30 minutes of outdoor light is more effective than hours under office fluorescents.
If you can't get outside (weather, schedule, or destination), a light therapy lamp or portable light therapy glasses can deliver 10,000 lux and are a reasonable substitute. They're particularly useful in the early morning when you need light exposure but it's still dark outside.
Sunglasses as a tool
This might seem odd, but sunglasses are a legitimate jet lag management tool. When you need to avoid light (for example, bright evening light after eastbound travel), wearing sunglasses indoors in a brightly lit environment reduces the signal to your brain.
Some frequent travellers carry both clear and dark glasses: clear for light-seeking windows, dark for light-avoiding windows. It's not glamorous, but it works.
Blue light
Your circadian clock is most sensitive to blue-wavelength light (around 480nm). This is why blue light from screens gets attention. But in practice, the intensity of the light matters far more than the wavelength. Sunlight contains plenty of blue light naturally. A bright room with warm-toned lights still delivers enough blue light to register as daytime.
Don't overthink the blue light angle. Focus on overall brightness: bright during your "seek" window, dim during your "avoid" window.
What the planner does with this
The Beat Jet Lag planner uses your flight times, direction of travel, and sleep habits to generate a personalised light exposure schedule, telling you exactly when to seek light and when to avoid it on each day of your trip.