Light is the main signal that sets your circadian rhythm. Specialised cells in your retina called ipRGCs (intrinsically photosensitive retinal ganglion cells) don't help you see shapes; they measure how bright your surroundings are, and they're most sensitive to blue-cyan light. Their signal tells your brain's master clock what time of day it is.
Andrew Huberman often stresses getting outdoor daylight within an hour of waking, and it's one of the most useful habits for sleep timing, mood and energy.
Why Does Weather Matter?
Light intensity is measured in lux. Outside on a clear morning, looking toward the sky (not at the sun), you're typically getting 10,000 lux or more. Under heavy overcast it can drop to around 1,000 lux, which is still brighter than most indoor lighting.
Morning light strengthens the morning cortisol rise and sets the timer for tonight's melatonin. The dimmer the sky, the longer it takes to get the same effect, so on cloudy days you stay out longer.
Rules for Morning Light
- Be Outside: Indoors, even next to a window, light is usually many times dimmer than outside, so step outside.
- Skip Sunglasses if You Can: Sunglasses cut the light that reaches your eyes. Clear prescription glasses or contacts are fine.
- Never Look at the Sun: Look around you or toward the sky, but never directly at the sun, which can damage your eyes.