Up to 80% of UV penetrates light cloud cover
Up to 80% of UV radiation can still penetrate light cloud cover, allowing some vitamin D production outdoors even on overcast days.
Snapshot · The Diary Of A CEO with Steven Bartlett
Vitamin D Expert: The Supplement World Is Giving The WRONG Advice!
Explore episode
Jul 27, 2026
Where this was said
How To Build The Right Sleep Protocol For Better Rest
At 1:25:56 · chapter starts 1:25:27
Bartlett raises the practical objection: he lives in London, where sunlight is scarce for much of the year. Dr. Gominak responds by explaining the melanin-sunlight adaptation story. Melanin protects DNA by absorbing UV energy — people with more melanin are protected at high-sun latitudes but produce less vitamin D per unit of exposure. The redhead gene, which arose in northern Viking-age Scotland, reduced melanin across most of the skin, leaving only freckles, specifically to maximise D production in weak northern sunlight. Transplant those genetics to Texas, however, and the lack of melanin becomes a hazard in high-UV environments. Bartlett then cites USA Today data: 90% of Americans spend close to 22 hours indoors daily, and 20% never go outside at all. He also presents cloud-cover data: up to 80% of UV can penetrate light cloud, but dense storm clouds block 70–90% of UVB, making meaningful D production nearly impossible. Dr. Gominak confirms that windows block UVB entirely, so sitting by a window produces no vitamin D. She notes UVA tanning beds tan the skin but do not produce D — only UVB-emitting beds do. She also makes a strong point about infrared radiation: simply sitting on a porch outside, even in shade, delivers infrared energy to the mitochondria, and incandescent bulbs (now largely replaced by LEDs) replicate this wavelength in a way LED lights cannot.
The Right Sleep Protocol Step By Step
Get your vitamin D blood level above 60. Take B50 for 3 months to restart the gut microbiome. Drop to a multivitamin. Track your sleep like a journal. When sleep breaks down again, your brain is asking for something specific — interpret it, don't give up.
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