Tip · Light

Midday sun for vitamin D

Midday sun provides UVB – the only source of vitamin D produced by the body itself.

Vitamin D is produced in the body in only one way: UV-B hits the skin. That is established photochemistry. But the tip has a downside that no other light tip has — the same radiation is the most important risk factor for skin cancer, and the dose is decisive.

In short

Yes, midday sun provides UV-B, and without UV-B the body does not produce vitamin D. With regular time spent outdoors, 80 to 90 percent of the supply comes from this production in the body. The robust dose statement, however, is not a number of minutes but a relative measure: half the minimal erythema dose of UV (0.5 MED) on the face, hands and arms, two to three times a week. For skin type II at a UV index of 7 that is about 12 minutes; for every other skin type this time is wrong. From October to March, the sun in Germany is not sufficient anyway.

What lies behind it

Synthesis is triggered by UV-B primarily in the 290–315 nm range. The photons hit sterol precursors such as 7-dehydrocholesterol in the epidermis and generate previtamin D intermediates; this is followed by thermal isomerization to vitamin D. Because it is a non-enzymatic photoreaction, it needs exactly this wavelength range — visible light and UV-A cannot trigger it.

The reaction is self-limiting. Sustained irradiation increasingly generates lumisterol, tachysterol and other photoproducts and thereby limits net vitamin D production. Longer in the sun therefore does not mean proportionally more vitamin D.

What the dose depends on

The UV-B share at ground level depends heavily on the position of the sun; according to the German Federal Office for Radiation Protection, the UV index changes most with season, time of day and geographical latitude. What matters is therefore not the clock time but the position of the sun — the same clock time in December provides nothing in Germany.

The consensus German recommendation therefore formulates the dose in relative terms: according to current knowledge, for sufficient vitamin D synthesis it is enough to expose the face, hands and arms, uncovered and without sun protection, two to three times a week to half the minimal erythema dose of UV. The often-quoted 12 minutes are an example value for skin type II at a UV index of 7.

The downside in numbers

Solar radiation, UV radiation and UV-emitting tanning devices are each classified by the IARC in Group 1, that is, as carcinogenic to humans; the monograph considers the causal link between cutaneous melanoma and sun exposure to be established. The Norwegian cohort puts numbers on this: compared with the trajectory “stable low”, the hazard ratios for a stable high sunburn frequency were 1.50 (95 % CI 1.28–1.75) for melanoma and 1.51 (1.22–1.87) for squamous cell carcinoma. Those who later reduced their sunburns remained at increased risk, at 1.44 and 1.47 respectively.

What is well supported

Three things hold up. First, the physiology: UV-B in the 290–315 nm range is the only route to vitamin D produced by the body, and that is not a hypothesis but described photochemistry. Second, the share: with regular time outdoors, 80 to 90 percent of the supply comes from the skin, not from food; the estimated value in the absence of the body’s own synthesis is 20 micrograms per day, and a serum level of at least 50 nanomoles per liter is considered sufficient for bone health. Third, the dose: a jointly supported recommendation brings both opposing goals together in a single, quantifiable statement.

What the studies show

The consensus German recommendation (BfS / UV Protection Alliance, as of August 4, 2026)

Not a single study but a jointly supported recommendation by several scientific authorities, professional societies and associations. It is authoritative here because it frames the dose in relative terms: half the minimal erythema dose of UV, two to three times a week, on the face, hands and arms. In addition, two clear statements: sunburn is to be avoided in principle, and strong UV exposure that is not medically supervised for the purpose of vitamin D production is strongly discouraged.

Sunburns over the lifespan (Lergenmuller et al. 2022)

Population-based prospective cohort from the Norwegian Women and Cancer Study, 172,472 women aged 31 to 70, of whom 169,768 provided information on sunburns. Mean follow-up 14.3 to 19.5 years; 1,252 to 1,774 melanomas and 739 to 871 squamous cell carcinomas were recorded. The two trajectory types with a high sunburn frequency at a young age consistently showed significantly increased risks.

What a supplement does not achieve (VITAL 2019, D-Health 2025)

VITAL: 25,871 participants, men aged 50 and over and women aged 55 and over, 2,000 IU vitamin D3 daily versus placebo, median follow-up 5.3 years. Both primary endpoints were missed: cancer HR 0.96 (95 % CI 0.88–1.06; p = 0.47), cardiovascular event HR 0.97 (0.85–1.12; p = 0.69). D-Health: 21,315 Australians aged 60 to 85, 60,000 IU monthly for up to five years, double-blind. No difference in cancer incidence: HR 1.02 (95 % CI 0.95–1.10), and no effect on fractures either.

What the recommendation does not say

The widespread statement of 10–20 minutes at midday is not a dose that can be substantiated. The consensus recommendation deliberately works with a relative measure, because the required time depends on skin type and the current UV index; the stated approximately 12 minutes apply only to skin type II at a UV index of 7. That midday is the best choice has not been tested either: the German Federal Office for Radiation Protection even recommends avoiding the midday hours outdoors at very high values of 8–10, and the recommendation does not name any time of day at all. And the formula “face, arms, legs are enough” deviates: the recommendation and the DGE name face, hands and arms.

The link between UV dose and melanoma is moreover not clear-cut in all analyses. A worldwide analysis of melanoma incidence against personal UV dose in more than 50 countries found no significant correlation, and among Europeans of skin types I–IV even one in the opposite direction. That is an ecological analysis with the weaknesses typical of such analyses — but it shows that the link is not as smooth as it is often presented.

How to do it

The robust version does not come from a study but from the consensus recommendation of the authorities. How much skin: face, hands and arms uncovered. Without sun protection, but only for this short time — for longer stays, protective measures are to be taken. How often: two to three times a week. How long: half the minimal erythema dose of UV, so about 12 minutes for skin type II at a UV index of 7. Longer at a lower UV index, longer with darker skin, shorter with lighter skin.

The UV index provides orientation. From a value of 3, protective measures are necessary according to the German Federal Office for Radiation Protection. And from October to March, the sun in Germany is not sufficient — the body then draws on the stores built up in summer.

Safety

This is the tip in this group with the most real risk, and the data on it are good. Sunburn is to be avoided in principle — the Norwegian cohort shows why: a high sunburn frequency over the lifespan was associated with risks increased by 44 to 51 percent, and later reduction did not eliminate the risk. Infants should, in principle, not be exposed to direct sunlight. Strong UV exposure that is not medically supervised, from the sun or a solarium, for the purpose of vitamin D production is strongly discouraged; in Germany, solarium use is prohibited for minors. For light skin types, many pigmented moles, a personal or family history of skin cancer, immunosuppressive therapy and photosensitizing medications, the weighing is a medical one. This page provides information, not a treatment recommendation and not a diagnosis.

BK-Score Well supported

Human evidence		8
Mechanism		9
Safety data		7
Hype gap		6
Track record of use		10

That UVB produces vitamin D in the skin is established physiology. The overall direction is mixed because of the downside: the same radiation is the most important risk factor for skin cancer – the dose is decisive, and there is no universally valid number for it.

What is rated is the state of knowledge, not the effect. “Safety data 9” means well studied – not harmless.
Subjective assessment by Biohacking Kompakt based on published scoring rules – not a scientific rating and not a medical recommendation. Rules and all ratings (German)

Frequently asked questions about midday sun for vitamin D

How long do I need to be in the sun for enough vitamin D?

The consensus German recommendation does not give a fixed number of minutes but half the erythema dose on the face, hands and arms, two to three times a week. For skin type II at a UV index of 7 that is about 12 minutes. With a different skin type or a different UV index this time changes, and there is therefore no universally valid number.

Does it also work behind glass?

Vitamin D is produced exclusively by UV-B in the range around 290–315 nm, and glass strongly filters precisely this short-wave portion. A series of measurements on 34 vehicles concluded that most of the cars studied offer effective UV-A and UV-B protection through the windshield. A corresponding series of measurements for window glass in homes is not known.

Is the sun enough in winter?

Not in Germany. The German Nutrition Society states that solar irradiation from October to March is not strong enough to ensure sufficient production. During this time the body draws on the stores built up in summer.

Isn’t a supplement simpler?

For bone health, the estimated value in the absence of the body’s own synthesis is 20 micrograms per day. For the more far-reaching promises, by contrast, there are two large negative studies: VITAL with 25,871 participants missed both primary endpoints, with HR 0.96 for cancer and HR 0.97 for cardiovascular events, and D-Health with 21,315 participants found no effect on cancer incidence, with HR 1.02. The question of sun or capsule is therefore less decisive than often presented.

Does sunscreen block vitamin D production?

The consensus recommendation assumes that the short exposure time takes place without sun protection, but that protection should be used for longer stays. There is no robust quantification of how much commercially available sunscreens reduce the production of 25(OH)D under everyday conditions. There are studies on this with contradictory results and different application amounts.

How dangerous is the midday sun really?

Solar radiation is classified by the IARC in Group 1, that is, as carcinogenic to humans, and the monograph considers the causal link between melanoma and sun exposure to be established. In the Norwegian cohort with more than 170,000 women, a life course with many sunburns was associated with hazard ratios of 1.50 for melanoma and 1.51 for squamous cell carcinoma. The dose is decisive: stay below the sunburn threshold.

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Information only, not medical advice and not a usage recommendation. If you have pre-existing conditions, and before major changes, consult a physician. Last updated: 2026-09-13.