Sunlight and vitamin D

LGT

None vitamin D made through a sunny window

Glass blocks the ultraviolet wavelengths that start vitamin D synthesis, even when the room feels bright and warm.

Contested, tiered honestly

Stasis Home Research Every claim on this page is graded against peer-reviewed health research and linked to its primary source.Updated August 2026

Sunlight through a window does not give you vitamin D, because glass blocks UV-B, the narrow band that starts synthesis in skin. The warmth and brightness that do come through are not substitutes, and the UV Index is a sunburn number rather than a vitamin D one. Ultraviolet is a carcinogen, so the sun is worth taking in moderation and at gentler hours, never chased for a number.

Sunlight and vitamin D: what is it, and where does it come from?

Sit in a bright window for an hour and your skin will feel warm, the room will read as sunny, and you will make no vitamin D at all. Ordinary window glass blocks UV-B, and UV-B, roughly 290 to 320 nanometres, is the only part of sunlight that starts the reaction in skin. Visible light does not do it and warmth does not do it, which is why a glass office and a sunny car produce nothing either.

Outdoors, the gate is not how bright the day looks but how high the sun sits. Low sun means a long slanted path through ozone, and ozone absorbs the vitamin D wavelengths, so the available window follows latitude and season rather than the weather.

The UV Index does not close that gap. It is defined on the action spectrum for sunburn, and sunburn-weighted dose is a poor predictor of vitamin D synthesis, so it reads burn risk and says nothing about what your skin is making.

Ultraviolet radiation is a carcinogen and the most preventable cause of skin cancer, which is why the useful move is moderation and timing rather than guesswork.

The UV Index is the right instrument for that even though it is the wrong one for vitamin D: it marks when protection starts at 3, when ultraviolet tops out, and when it falls back under 3. The hours outside that window are the gentler ones.

Sunlight and vitamin D: why does it matter?

You want to know how much sun you need, and nobody can tell you. The blood response to the same ultraviolet dose varies more than fortyfold between people, which makes every minutes-per-day formula false precision wearing the costume of guidance.

Geography is the part that is knowable. Above roughly 42 degrees north there is a winter stretch when meaningful synthesis stops no matter how the sky looks, while below about 35 degrees there is largely no such stretch. The study that established it tested four places and the pattern is clean: winter sunlight in Boston made none, Edmonton went dark for longer, and Los Angeles and Puerto Rico ran all year.

None of it converts into a personal dose, and none of it changes the sunscreen answer: correct sunscreen use does not prevent vitamin D synthesis and does prevent burning, so there is never a reason to skip it.

The ceiling sits on the supplement side. The large trials were null in people who were already replete. VITAL randomised 25,871 people to 2,000 IU a day for a median 5.3 years with no reduction in cancer or major cardiovascular events, a fracture analysis from the same trial was null, and so was a meta-analysis of 21 randomised trials and 83,291 participants.

Mechanism and latitude are knowable. Your personal dose is not, and neither is a promise on the far end of it.

Skin makes vitamin D from UV-B, roughly 290 to 320 nanometres. Visible light and warmth do not start the process, and glass blocks UV-B, so a sunny window produces none.NIH Office of Dietary Supplements, Vitamin D Fact Sheet for Health Professionals
The UV Index is defined on the action spectrum for sunburn, not vitamin D.WHO, WMO, UNEP and ICNIRP, Global Solar UV Index: A Practical Guide
Sunburn-weighted dose is a poor predictor of vitamin D synthesis, so the UV Index is not a vitamin D readout.Young et al., 2021 (PNAS; a revised action spectrum for vitamin D synthesis by suberythemal UV radiation exposure in humans in vivo)
Above roughly 42 degrees north, winter includes a stretch with no meaningful skin synthesis; below about 35 degrees there is largely no such stretch. This is a latitude pattern: it says when synthesis is possible, not how long anyone should be outside.Webb et al., 1988 (J Clin Endocrinol Metab; winter sunlight in Boston and Edmonton does not promote vitamin D3 synthesis in skin)
The study that established this tested four places. Winter sunlight in Boston produced no vitamin D in skin, Edmonton's dead stretch was longer still, and Los Angeles and Puerto Rico supported synthesis year round.Webb et al., 1988 (J Clin Endocrinol Metab; winter sunlight in Boston and Edmonton does not promote vitamin D3 synthesis in skin)
Correct sunscreen use did not prevent vitamin D synthesis and did prevent burning, so there is never a reason to skip sunscreen for vitamin D.Young et al., 2019 (British Journal of Dermatology; correct sunscreen use and vitamin D status)
VITAL randomised 25,871 people to 2,000 IU a day for a median 5.3 years and found no reduction in cancer or major cardiovascular events. The large trials were null in people who were already replete, and that dose describes what the trial did, not a recommendation.VITAL randomised trial, 2019 (NEJM; vitamin D supplementation and the prevention of cancer and cardiovascular disease, n = 25,871)
The VITAL fracture analysis was also null in people who were already replete.LeBoff et al., 2022 (NEJM; supplemental vitamin D and incident fractures, VITAL ancillary study)
A meta-analysis of 21 randomised trials and 83,291 participants found no cardiovascular benefit from supplementation.Barbarawi et al., 2019 (meta-analysis of 21 randomised trials and 83,291 participants; vitamin D supplementation and cardiovascular outcomes)

Sunlight and vitamin D: What do these numbers actually describe?

What the research reports about ultraviolet light and vitamin D, and what each figure is.
What was studied	What the research reports	Source
Vitamin D-producing ultraviolet band	About 290 to 320 nm UV-B starts synthesis in skin; visible light and warmth do not	NIH Office of Dietary Supplements, accessed 2026
Vitamin D through window glass	None glass blocks UV-B, even when the window looks bright	NIH Office of Dietary Supplements, accessed 2026
UV Index as a vitamin D measure	Not valid the index is weighted for sunburn, which poorly predicts vitamin D synthesis	Young et al., 2021
Winter synthesis by latitude	Stops above roughly 42° north below about 35° north there is largely no winter stop; latitude and season, not a personal dose	Webb et al., 1988
Blood response to the same ultraviolet dose	More than fortyfold spread measured across people given identical UV-B doses; too variable to turn into a personal time or dose	Bogh et al., 2016

Every line here is a published finding or a physical fact about ultraviolet light. None of it is a limit, and none of it is a dose. There is no legal limit on sunlight, and there is no health threshold your report grades you against.

What helps against Sunlight and vitamin D?

4 approaches below reduce Sunlight and vitamin D in a home, ordered from the most direct fix to the smallest change in habit. The most direct is Treat sunny windows as light, not vitamin D. Each entry also names what it cannot do, because no single fix covers every exposure route.

Free and behavioral

Treat sunny windows as light, not vitamin D
Use the window for brightness and warmth without counting it as vitamin D exposure. Glass blocks the UV-B that starts synthesis in skin.
Outdoors is a separate decision, and the UV Index is the instrument for it: protection starts when it reaches 3, and the gentler hours sit outside the day's peak.
Keep using sunscreen correctly
Correct sunscreen use does not prevent vitamin D synthesis and does prevent burning. There is never a reason to skip it for vitamin D.
No sunscreen makes ultraviolet exposure harmless. Ultraviolet radiation is carcinogenic and the most preventable cause of skin cancer.
Read latitude as the calendar for synthesis
Latitude and season explain why the vitamin D window changes. Above roughly 42 degrees north there is a winter stop, while below about 35 degrees there is largely none.
It tells you when synthesis is possible where you live, not how long to be out. No latitude sets a personal dose.
Reject minute formulas and UV Index dosing
The same ultraviolet dose produces more than a fortyfold spread in blood response, and the UV Index measures sunburn rather than vitamin D.
Nobody can turn those inputs into your personal number.

This is where wellness advice oversells hardest, and the oversell always arrives in the same shape: a number of minutes, delivered with total confidence. We can give you the wavelengths, the glass, and the latitude pattern, because those are real and they are stable.

We cannot give you a personal ultraviolet prescription, because the same dose produces a more than fortyfold spread in blood response and that spread does not collapse for anyone. What we can give you is the shape of the day.

Ultraviolet radiation is a carcinogen and the most preventable cause of skin cancer, so the honest posture is moderation and timing: take the sun in the gentler hours, use the UV Index to see when protection starts at 3 and when the day peaks, and keep the sunscreen on, since correct use does not cost you any synthesis.

Common questions about Sunlight and vitamin D

What is an action spectrum, and why does it separate sunburn from vitamin D?

An action spectrum is the map of which wavelengths drive a particular biological effect, and how strongly each one does it. Sunburn and vitamin D synthesis do not share one, so a single ultraviolet reading cannot serve both. The UV Index is built on the sunburn spectrum, and sunburn-weighted dose turns out to be a poor predictor of vitamin D synthesis.

Why does how high the sun sits decide whether UV-B reaches you at all?

Sun height sets how much atmosphere the light has to cross. Low sun means a long slanted path through ozone, and ozone absorbs the vitamin D wavelengths, so the UV-B thins out before it reaches you. High sun means a shorter path and less absorbed. Brightness and warmth are poor guides here, because visible light and heat get through when UV-B does not.

What did Webb 1988 actually measure?

It tested four locations. Winter sunlight in Boston produced no vitamin D synthesis in skin, Edmonton's dead stretch was the longer of the two, and Los Angeles and Puerto Rico supported synthesis year round. That is the whole geographic result: above roughly 42 degrees north winter includes a stretch with no meaningful synthesis, and below about 35 degrees there is largely none of that stretch.

Does the 42 degree line tell you anything about a city Webb did not test?

The 42 degree line is a latitude band, not a city list. Webb tested four places: winter sunlight in Boston and Edmonton produced no synthesis in skin, Edmonton's stretch the longer, while Los Angeles and Puerto Rico ran year round. Above roughly 42 degrees north winter includes a stretch with no meaningful synthesis; below about 35 degrees, largely none. A month count for an untested city is invented.

What does a fortyfold spread in blood response mean in practice?

In a study giving identical UV-B doses to people of similar skin pigmentation, the rise in blood vitamin D varied more than fortyfold across individuals. That is why nobody can hand you a number of minutes. The same ultraviolet input produces very different results in different bodies, so a population average tells you close to nothing about yourself.

Why were the VITAL results null?

VITAL randomised 25,871 people to 2,000 IU a day for a median 5.3 years and found no reduction in cancer or major cardiovascular events. Its fracture analysis was null, and so was a separate meta-analysis of 21 randomised trials and 83,291 participants. The people enrolled were largely already replete, so what got tested was topping up people who were not short.

How many trials came back null, and what did they enrol?

VITAL was not the only null result. A fracture analysis from that same trial was null, and so was a meta-analysis of 21 randomised trials and 83,291 participants. VITAL randomised 25,871 people to 2,000 IU a day for a median 5.3 years with no reduction in cancer or major cardiovascular events. All tested topping up people already replete, and that dose describes what a trial gave, not a recommendation.

What does 'already replete' mean?

It means the people in the trial already had enough vitamin D on board before anything was given to them. That matters because it sets the limit of what a null result can say. Those trials tested adding more to people who were not short, and they are silent about anything else. Reporting that plainly is a different thing from promising a benefit at the other end.

Did the sunscreen study test correct use or ordinary use?

It tested correct use, properly applied, which is the condition that makes the result meaningful. Under it, sunscreen did not prevent vitamin D synthesis and did prevent burning. So the trade people imagine, a little burning accepted in exchange for a little vitamin D, is not a real trade. There is never a reason to skip sunscreen for vitamin D.