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Indoor vs Outdoor Light
Why windows aren't enough
In short
Next to a window you get about a tenth of the light you would get outside on the same cloudy day.
On an overcast day outdoors, you might experience 5,000–10,000 lux of diffuse illumination. Indoors next to a window on the same day? Perhaps 500–1,000 lux. A typical well-lit office? 300–500 lux.
This isn't just a number — it represents an order-of-magnitude difference in the strength of the circadian signal reaching your eyes.
Standard glass also filters significant UV and some near-visible wavelengths, further attenuating the spectral profile that your circadian system evolved to read.
Research suggests that most people in modern industrialized environments receive substantially less morning and daytime light exposure than would be ideal for robust circadian function — and substantially more evening light than is conducive to sleep.
Sources
- Recommendations for daytime, evening, and nighttime indoor light exposure to best support physiology, sleep, and wakefulness in healthy adults — Brown et al. (2022), PLOS Biology
- Entrainment of the Human Circadian Clock to the Natural Light-Dark Cycle — Wright et al. (2013), Current Biology