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Westward daylight still counts
Why overcast light works
The trigger is the sheer number of photons, and the sun itself does not have to be in view. At 9 AM on an overcast winter day facing west, it is still far brighter than at midnight the night before. That brightness fires the intrinsically photosensitive melanopsin retinal ganglion cells, which connect directly to the circadian clock.
Morning chemistry
Morning light raises the early cortisol peak and the levels of dopamine, epinephrine, and norepinephrine, which means more energy, mood, and focus through the day. Spiking these alerting chemicals early leaves their levels low at night. Cortisol runs in opposite phase to melatonin, so natural melatonin release rises in the evening, and falling asleep and staying asleep get easier.
Better and worse light
A clear day beats an overcast one, and facing east beats facing west, though west still works. Sunglasses and brimmed hats cut the light arriving from above, while eyeglasses and contacts do not. When the sun hasn't risen yet, a 10,000 lux artificial light can stand in. Light so bright it is painful to look at without blinking is too bright, and the blink reflex exists to protect the retina.
How do melanopsin cells sense brightness?
David Berson's lab at Brown University reported these cells in a 2002 paper in Science. They contain the pigment melanopsin, which responds most strongly to blue light around 480 nanometers. They measure overall brightness, not images, and send that signal along the retinohypothalamic tract to the suprachiasmatic nucleus. They make up only a small fraction of the retina's ganglion cells.
Why does overcast daylight still count as bright?
Clouds scatter sunlight but block less of it than they seem to. Outdoor light on an overcast day typically measures in the thousands of lux, and a clear sunny day can exceed 100,000 lux. Ordinary indoor lighting usually measures a few hundred lux. Night light from the moon is below one lux.
Why does cortisol surge after waking?
Researchers call the surge the cortisol awakening response. Jens Pruessner and colleagues described it in 1997 in Life Sciences, measuring salivary cortisol in the first hour after waking. Levels climb for roughly 30 to 45 minutes and then decline across the day. The surge is thought to help prepare the body for activity.
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