Light timing for travel

Updated 2026-08-17

Light is the strongest input your body clock has — stronger than melatonin, meal times or willpower. At the right hour it moves you a full step in a day. At the wrong one it moves you backwards.

The rule

Everything hinges on CBTmin, roughly two hours before your habitual wake time.

Light after it → clock moves earlier → good for flying east
Light before it → clock moves later → good for flying west

How much light, and for how long

Brightness is measured in lux, and indoor lighting is far dimmer than it feels:

Setting	Roughly
Living room at night	50–200 lux
Well-lit office	300–500 lux
Overcast day, outdoors	1,000–10,000 lux
Direct sun	50,000+ lux

Which is why twenty minutes outside beats an hour by the window, even in bad weather. If you take one thing from this: go outside, don’t sit near glass.

When the sun isn’t up at the hour you need it, a 10,000-lux light box at arm’s length for 20–30 minutes is the standard substitute. This matters in winter at high latitudes, where a plan asking for morning light at 06:00 is asking for something that doesn’t exist yet.

Avoiding light is half the job

The dark half gets ignored. Flying west, bright morning light drags you back east and undoes the previous night — sunglasses on the walk to breakfast are doing as much work as the evening light later.

Screens matter far less than headlines suggest. A phone at arm’s length is a few tens of lux. It’s not nothing, but it’s not the sun, and blaming screens while walking into daylight at the wrong hour is missing the actual lever.

In the app

Ray checks the real sunrise and sunset where you are before telling you to step outside. When the sun isn’t up yet, the cue says lamp or light box instead — because “get morning light” at 6am in December is advice that can’t be followed.