Why it's not 100% useless - and what this means for indoor workers.
I used to say that sunlight through a window “doesn’t really count"....
And for a long time, that was a useful simplification, but a brand new human study made me pause, and then completely rethink how I talk about daylight, windows, blood sugar, and metabolism.
Keep reading to learn more about the study & what it means for indoor workers
For years, I’ve said that sunlight through a window doesn’t “count.”
And from a classical circadian signaling standpoint - UVB, vitamin D, full-spectrum infrared that’s still true (you cannot make vitamin D from behind a window & miss out on a ton of benefits).
But new research shows something important I don’t think the circadian health space has fully integrated yet: Light through a window may not be complete - but it’s not meaningless.
First let's be clear about what a window DOES block.....
This new research made me pause - not because it contradicts what we know about UV, circadian signaling, or lux - but because it highlights something we’ve been overlooking.
Incomplete signals can still be biologically meaningful.
The study showed that people exposed to natural daylight during office hours had measurable improvements in glucose control and whole-body substrate metabolism - even though that light was filtered through windows, reduced in intensity, and missing ultraviolet wavelengths.
If daylight through glass truly “did nothing,” we wouldn’t see those changes.
So instead of asking “does window light replace outdoor sunlight?” - which it clearly does not - the better question is:
What signals are still getting through, and what do they do?
Windows block UV-driven circadian and melanocortin signaling, and they dramatically reduce lux. That means indoor daylight cannot fully anchor the circadian system, drive vitamin D production, or deliver the same metabolic clarity as time spent outdoors.
But windows do not block visible light - and importantly, they do not block most red wavelengths.
And red light works through a completely different biological pathway.
Rather than acting through UV-dependent circadian and neuroendocrine signaling, red light acts at the level of the mitochondria - improving electron transport efficiency, reducing oxidative stress, and supporting cleaner glucose handling (and better blood sugar regulation overall).
This also tracks very well with the work of Dr. Glenn Jeffrey (shown below): long-wavelength red light improves mitochondrial efficiency and can lower post-meal glucose spikes by reducing cellular energy stress. Importantly, this effect does not depend on UV exposure or vitamin D.
To read the full deep dive I did on this - including my "Indoor office worker with no windows" protocol - click here!
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(The contest winners from last week were Jill McMahon & Susan Gaines - and they have both been sent their winnings!)
Thanks for being here & I hope this article was illuminating!
