Insights on Vitamin D, sun health, and the science behind safe sun exposure
In theory sunscreen blocks vitamin D, since SPF 30 filters about 97% of UVB. In practice, people apply too little and miss spots, so studies consistently find regular sunscreen use does not cause deficiency.
No. Ordinary window glass blocks almost all UVB, the wavelength your skin needs to make vitamin D, so a sunny window does next to nothing for your levels, while still letting through skin-aging UVA.
Yes, you can still make some vitamin D on a cloudy day, but a lot less. Full overcast can cut UV reaching the ground to about a third, slashing your vitamin D production by roughly two-thirds.
The best time to make vitamin D is the middle of the day, roughly 10am to 3pm and centered on solar noon, when the sun is highest and UVB is strongest. Morning and late-afternoon sun produce little to none.
Your skin only makes vitamin D when the UV index hits 3 or higher, because below that the sun sits too low for UVB to reach the ground. Here is why 3 is the threshold and how to use it.
Most people make enough vitamin D from 10-30 minutes of midday sun, but the real number depends on the UV index, your skin tone, how much skin is bare, and your latitude. Here is how to find yours.
Compare the best Vitamin D tracking apps in 2026: Dminder, SunSafe, Rays, and Sun Day, plus why Bask stands out with UV/weather forecasts, Apple Health daylight tracking, safe sun windows, supplements + cofactors, and physician-ready exports.
Learn how your skin produces Vitamin D from sunlight, the factors that influence synthesis, and how to optimize sun exposure safely based on scientific research.