How Much Sun Do You Need for Vitamin D in May?

Vitamin D is something your body can produce on its own, but it needs UVB radiation hitting your skin at the right angle. May is when Central Europe finally enters the season where spring sunlight is strong enough to trigger vitamin D production in your skin, but timing, exposed surface area, and skin type all play a role.

Why can't you make vitamin D in winter and early spring, even when the sun is shining?

Your skin can only produce vitamin D when it is exposed to UVB radiation with a wavelength between 290 and 315 nm. During winter and early spring, the sun sits too low in the sky, and most UVB rays scatter in the atmosphere before they ever reach your skin.

This phenomenon is known as vitamin D winter. According to an analysis by Khanna et al. (2022), which processed data from across Europe, the vitamin D winter at latitudes 48° to 49°N (roughly covering cities like Vienna, Munich, Prague, or Budapest) lasts from October to March. During these months, the UVB radiation reaching the skin is so weak that vitamin D production is virtually zero, even on sunny days. [R]

This means that even if the sun was shining in February and you spent time outdoors, your body did not produce a single microgram of vitamin D. The reason is not the intensity of light itself but the angle at which the sun's rays hit the Earth's surface.

What is the solar angle and why does it determine your vitamin D production?

The solar zenith angle (SZA) is the angle between the sun and the vertical line directly above your head. The lower the sun is on the horizon, the greater the SZA, and the longer the path UVB rays must travel through the atmosphere. When the SZA exceeds roughly 50 to 55°, the atmosphere filters out virtually all UVB radiation, and vitamin D production in the skin comes to a halt.

Research by Neville et al. (2021) from University College Cork confirmed that UVB radiation makes up less than 5% of total UV radiation from the sun, yet this small fraction is the only one that triggers the conversion of 7-dehydrocholesterol into previtamin D3 in the skin. If the SZA is too large, UVB simply cannot penetrate the ozone layer. [R]

In practical terms, for latitudes around 48 to 50°N (Central Europe, northern United States, southern Canada), this means:

• November to February: the SZA stays above 55° all day, so no vitamin D production occurs
• March: the SZA drops below 55° only briefly around solar noon, and production is minimal
• April: the SZA falls below 50° between 11:00 and 13:00, marking the start of the vitamin D season
• May: the SZA stays below 40° from 10:00 to 15:00, enabling full vitamin D production

When in May does the sun finally provide enough UVB for vitamin D?

In May, Central Europe opens up a 5 to 6 hour window during the day when UVB radiation is strong enough for vitamin D synthesis. This window spans roughly from 10:00 to 15:00 or even 16:00 local summer time on clear days.

According to the global study by Kift et al. (2024) from the University of Manchester, vitamin D synthesis at latitudes above 40°N is limited to certain months of the year. At 48 to 49°N, the vitamin D season runs approximately from April to September, with the peak falling in June and July. [R]

May is therefore the first month when you can reliably stock up on vitamin D from sunlight. After a long vitamin D winter, this is exactly the time to start going outside and exposing your skin, ideally from 10:00 onward on clear days.

Keep in mind, however, that cloud cover matters. Thick cloud layers can reduce UVB intensity by 50 to 80%, which in practice means that on an overcast May day your body may not produce enough vitamin D, even though the time and sun angle are correct.

How many minutes of sun exposure do you need for your daily dose of vitamin D?

To produce an adequate daily dose of vitamin D in May, most people need 15 to 30 minutes of direct sunlight on exposed skin between 10:00 and 14:00, provided skies are clear and at least 35% of the body surface is uncovered (hands, forearms, face, neck, and ideally legs).

This figure comes from a review by Wacker and Holick (2013) published in Dermato-Endocrinology, which showed that brief sun exposure covering the hands, face, and legs for 5 to 30 minutes twice a week (depending on skin type) can produce sufficient vitamin D. [R]

The exact time depends on several factors:

It is important not to exceed the minimal erythemal dose (MED), which is the point at which your skin starts to turn red. The goal is not a sunburn but rather a suberythemal dose, where the body produces vitamin D while simultaneously building its melanin defence.

What blocks vitamin D production in the skin?

Vitamin D is synthesised directly in the skin through UVB radiation, yet several common factors can significantly reduce or completely block this process, even when you are outside at the right time.

Clothing and glass. Standard window glass (car windows, office windows) blocks virtually 100% of UVB radiation. If you are sitting behind a window, you will not produce vitamin D. Similarly, clothing, even a thin shirt, captures most UVB rays before they reach the skin.

Chemical sunscreens. Sunscreens with SPF 30 and above reduce vitamin D production in the skin by 95 to 99% according to research. Instead of automatically applying SPF before every outing, consider gradual skin adaptation and sensible timing of your sun exposure. Chemical filters also penetrate the bloodstream and may disrupt the hormonal system. You can learn more about this approach in our article on what a solar callus is and how it protects against sunburn.

Air pollution. Smog and elevated ground-level ozone further reduce UVB intensity, particularly in cities. Urban residents may therefore have lower vitamin D production compared to people living in rural areas.

Age and melanin. As you age, the amount of 7-dehydrocholesterol in the skin decreases; this is the precursor to vitamin D. A 70-year-old has roughly one quarter of this precursor compared to a 20-year-old, according to Holick (2007). People with darker skin (higher melanin content) need longer exposure to produce the same amount of vitamin D, because melanin absorbs UVB radiation.

How can you build natural skin protection instead of using SPF?

Your skin has a built-in mechanism for gradual UV adaptation, commonly called a solar callus. This involves a thickening of the epidermis and increased melanin production, which makes your skin progressively more resistant to sunburn while still efficiently producing vitamin D.

Instead of automatically reaching for a sunscreen with chemical filters, try gradual adaptation:

1. Start in April and May with morning sunlight (before 10:00), when UV intensity is lower but still beneficial for your eyes and circadian rhythm.
2. Gradually extend your time in stronger midday sun, starting with 5 to 10 minutes and adding a few minutes each day.
3. Uncover your skin step by step, beginning with forearms and face, then legs and torso.
4. Never try to tan in a single day. Sunburn is the exact opposite of what you want to achieve.
5. If prolonged exposure is unavoidable (a beach holiday after months spent indoors), use physical protection: clothing, shade, a hat.

You can find more about gradual adaptation and building a solar callus in our article Solar callus: how to build your sun tolerance in May.

Your grandmother worked in the fields from spring to autumn and never got sunburned, because she built up this solar callus year after year. Today's average person spends 90% of their time indoors and arrives at the beach with pale, unadapted skin, completely defenceless against UV. The solution is not more SPF but a return to gradual, natural adaptation. Read more in our article Why didn't your grandmother get sunburned and you did?

A practical 14-day skin adaptation plan for May

If you spent the whole winter indoors and your skin is fair and unadapted, here is a concrete 14-day plan for safely building a solar callus in May:

Days 1 to 3: Go outside in the morning between 8:00 and 9:30. Expose your forearms and face. Spend 15 to 20 minutes outdoors. At this time, UVA is stronger than UVB, which helps your skin prepare for more intense radiation.

Days 4 to 7: Move closer to midday, between 10:00 and 11:00. Uncover your neck and upper chest as well. Stay outside for 10 to 15 minutes. Monitor your skin; if no redness appears, you are on the right track.

Days 8 to 14: Add midday sun, 11:00 to 13:00. Expose your legs (wear shorts). Extend your time in the sun to 15 to 25 minutes. Your skin should already show a noticeably darker tone and thicker texture, which is a sign your solar callus is developing.

After two weeks, you will be able to spend 30 minutes or more in the May sun without the risk of burning, while your body efficiently produces vitamin D, melanin, and endorphins. If even mild redness appears during this adaptation, take a step back and shorten your exposure time.

Can you produce vitamin D without sunlight using a UVB panel?

Yes. Clinical studies confirm that UVB radiation from an artificial source triggers the same photochemical reaction in the skin as sunlight, provided it emits wavelengths in the 290 to 315 nm range. On days when it is overcast or you cannot go outside, a UVB panel can fill the gap that the sun leaves behind.

A pilot clinical study published in Scientific Reports (2025) on a sample of volunteers confirmed that controlled UVB light exposure was able to raise serum vitamin D levels and alleviate vitamin D deficiency. [R]

This is where light therapy in the home comes into play. Mitochondriak® devices include not only red and near infrared light but also a UVB wavelength of 295 nm, which can stimulate the skin to produce vitamin D, melanin, and other important molecules. A single infrared panel can thus replace the sun on days when it is not shining. Specifically, the Mitochondriak® Maxi UVB Upgraded, currently available for pre-order with a 20% discount, was designed precisely for this purpose.

Practical tips for using UVB light on Mitochondriak® devices:

• Always combine UVB with red and NIR light (never use UV on its own)
• Start with 30 seconds of UV and gradually increase to a maximum of 3 minutes per session
• Use at least 2 hours after sunrise and no later than 2 hours before sunset
• Allow a minimum 2-hour gap between sessions
• Before your first use, test your photosensitivity following the instructions in our FAQ

For a detailed guide on how to use UV light with Mitochondriak® devices, read our article Vitamin D: why a tablet isn't enough and how to produce it naturally.

Which foods contain vitamin D?

Diet is a supplementary source of vitamin D, but food alone covers only a fraction of your daily requirement. Nonetheless, it is worth knowing which foods can help you maintain an optimal 25(OH)D level in May:

• Fatty fish (salmon, mackerel, sardines, herring): the richest dietary source; a single serving of salmon provides roughly 600 to 1,000 IU of vitamin D3
• Cod liver oil: one tablespoon delivers around 1,360 IU
• Egg yolks: one yolk contains approximately 40 IU; eggs from free-range hens may contain more
• Game and organ meats: liver contains measurable amounts of vitamin D, especially from pasture-raised animals
• Sun-dried mushrooms (shiitake, maitake): contain vitamin D2 if dried in sunlight

The key is to view food as a complement, not a substitute for UVB exposure. The recommended daily intake of vitamin D, according to various experts, is 1,000 to 4,000 IU, while a typical diet provides at most 200 to 400 IU per day. The rest must come from sunlight or a quality UVB source.

Frequently asked questions about vitamin D and sun exposure in May

Can I make vitamin D while sitting by an office window?

No. Standard window glass blocks virtually 100% of UVB radiation, which is essential for vitamin D synthesis in the skin. UVA rays do pass through glass, but they do not trigger vitamin D production. To produce vitamin D you need to be outdoors with your skin directly exposed to sunlight.

Is 10 minutes in the sun enough to top up my vitamin D in May?

For people with fair skin (skin type I and II), yes, provided at least 35% of the body is exposed between 10:00 and 14:00. People with darker skin or those over 65 may need 25 to 40 minutes. On overcast days the required time increases substantially.

Why shouldn't I apply SPF sunscreen before a short time in the sun?

Sunscreens with SPF 30 or higher block 95 to 99% of UVB radiation, which effectively halts vitamin D production. For short outings (below your MED), gradual skin adaptation is a better approach. Use sunscreen only for extended exposure on unadapted skin, such as a beach holiday after months spent indoors.

From which month can you produce vitamin D from sunlight at northern latitudes?

Real vitamin D production from sunlight begins at latitudes around 48 to 49°N in April (around solar noon), and the full season runs from May to September. From October to March, UVB radiation at these latitudes is too weak for vitamin D synthesis.

Is vitamin D from a supplement as effective as vitamin D from UVB light?

Not entirely. Supplemental vitamin D3 is absorbed through the digestive tract, whereas UVB exposure produces it directly in the skin alongside other beneficial molecules (melanin, endorphins, nitric oxide). A supplement is a safety net for winter months, but it cannot replicate the full spectrum of benefits that natural or quality artificial UVB light provides.

What should my blood level of vitamin D be?

The optimal level of 25(OH)D in the blood is, according to most experts in mitochondrial health, above 40 ng/ml (100 nmol/l). Many standard laboratories consider 30 ng/ml sufficient, but from a prevention and performance standpoint a higher level is more beneficial. After winter, it is wise to have your levels checked with a blood test.

Can you overdose on vitamin D from the sun?

No, you cannot overdose on vitamin D from sunlight. Your body has a built-in regulatory mechanism. Once enough previtamin D3 is formed in the skin, additional UVB radiation converts it into inactive metabolites. Overdose is only possible through excessive supplement intake.

What should I do on cloudy May days when the sun isn't shining?

If the sun is absent for several days in a row, you can top up vitamin D with a UVB panel at home. Mitochondriak® devices featuring the 295 nm wavelength can trigger the same photochemical reaction in the skin as sunlight. Another option is vitamin D-rich foods (fatty fish, egg yolks) and vitamin D3 supplements.

Summary

May is the first full month for vitamin D production from sunlight in Central Europe. The key is to get outside between 10:00 and 15:00, uncover at least 35% of your body, and avoid coating your skin with chemical SPF during short outings. Adapt your skin gradually, build your solar callus, and on overcast days turn to devices with UVB light.

Want to make the most of the spring vitamin D season? Check out our Mitochondriak® infrared panels with UV.

Scientific studies and references

1. Khanna T. et al. Comprehensive Analysis of Seasonal and Geographical Variation in UVB Radiation Relevant for Vitamin D Production in Europe. Nutrients. 2022. PMC9735494
2. Neville JJ. et al. Physical Determinants of Vitamin D Photosynthesis: A Review. JBMR Plus. 2021. PMC7839826
3. Kift RC. et al. Globally Estimated UVB Exposure Times Required to Maintain Adequate Vitamin D Status. Photochemistry and Photobiology. 2024. PMC11124381
4. Wacker M., Holick MF. Sunlight and Vitamin D: A global perspective for health. Dermato-Endocrinology. 2013. PMC3897598
5. Nature Scientific Reports. A pilot clinical trial to explore the effects of UV exposure on vitamin D status. 2025. doi:10.1038/s41598-025-09203-8
6. Kallioglu MA. et al. UV index-based model for predicting synthesis of (pre-)vitamin D3. Scientific Reports. 2024. PMC10861575