The vitamin D sun window: when is UV strong enough?
Sunscreen advice tells you when UV is too strong. This page answers the opposite question: when is it strong enough for your skin to make vitamin D?
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Checking the sun…
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| Hour | UV index | Level |
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👆 The "protection needed" window above (UV 3+) is also your vitamin D window — the same UVB that burns is the UVB that synthesizes vitamin D.
The UV 3 rule works in both directions
Vitamin D synthesis needs UVB, and UVB only reaches ground level in meaningful amounts when the sun is reasonably high in the sky — in practice, when the UV index is about3 or higher. Below that, you can sit outside all afternoon and make almost none. That has a few non-obvious consequences:
- Short midday beats long evening. 10–20 minutes with forearms and face exposed around midday does more than hours of weak slanted sun.
- You don't need to burn. Synthesis plateaus well before skin reddens. Sub-burn exposure is the whole game — get your minutes, then cover up or apply SPF.
- Darker skin needs more time. Melanin filters UVB, so Fitzpatrick type V–VI skin may need 3–5× the exposure of type I–II for the same synthesis.
- Glass blocks it. UVB doesn't pass through windows — a sunny desk doesn't count.
"Vitamin D winter"
Above roughly 35–40° latitude (north of Los Angeles or Athens, say), winter sun never gets high enough: the UV index stays below 3 for weeks or months, and skin synthesis effectively stops. If you live at those latitudes, the window on this page will simply show none in midwinter — that's the honest answer, and it's why many clinicians suggest supplementing in winter. Ask your doctor what's right for you; this page is general information, not medical advice.
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