Many people wonder whether late afternoon sun exposure can contribute to their vitamin D levels, particularly after 4pm when outdoor activities may be more convenient. Vitamin D synthesis depends critically on ultraviolet B (UVB) radiation from sunlight, which varies considerably throughout the day based on the sun's position in the sky. In the UK, the ability to produce vitamin D after 4pm is generally very limited, especially outside summer months. Understanding when and how effectively your skin can synthesise vitamin D helps inform decisions about sun exposure, dietary intake, and supplementation to maintain adequate vitamin D status year-round.

How Vitamin D Is Produced From Sunlight

Vitamin D synthesis in the skin is a complex photochemical process that depends on exposure to ultraviolet B (UVB) radiation from sunlight. When UVB rays penetrate the skin, they interact with 7-dehydrocholesterol, a cholesterol derivative present predominantly in the epidermis. This interaction converts 7-dehydrocholesterol into previtamin D₃, which then undergoes thermal isomerisation to form cholecalciferol (vitamin D₃).

Once produced in the skin, vitamin D₃ enters the bloodstream and is transported to the liver, where it is hydroxylated to form 25-hydroxyvitamin D [25(OH)D], the main circulating form used to assess vitamin D status. A second hydroxylation occurs in the kidneys, producing the active hormone 1,25-dihydroxyvitamin D [1,25(OH)₂D], also known as calcitriol. This active form primarily regulates calcium and phosphate metabolism, supporting bone health, while its roles in immune function and other cellular processes continue to be investigated.

The efficiency of this process depends critically on UVB availability. UVB radiation has a wavelength of 280–315 nanometres, and its intensity at ground level varies considerably based on several environmental factors. The sun's angle in the sky is particularly important—when the sun is low on the horizon, UVB rays must travel through more atmosphere, where they are absorbed and scattered by ozone, clouds, and air molecules. This means that the time of day, season, latitude, and weather conditions all significantly influence whether adequate UVB reaches the skin to initiate vitamin D synthesis.

Understanding these mechanisms helps explain why vitamin D production is not constant throughout the day and why certain times are more effective for maintaining adequate vitamin D levels through sun exposure alone.

Can You Get Vitamin D From the Sun After 4pm?

The ability to synthesise vitamin D from sunlight after 4pm in the UK is generally limited, particularly outside the summer months. This is primarily due to the sun's position in the sky during late afternoon and evening hours, which affects the intensity of UVB radiation reaching the earth's surface.

When the sun is at a low angle, UVB rays must pass through a significantly greater thickness of atmosphere. During this extended path, much of the UVB radiation is absorbed by atmospheric ozone and scattered by air molecules, meaning less UVB reaches ground level to trigger vitamin D production in the skin. In the UK, particularly from October through March, the sun's angle is typically too low for effective vitamin D synthesis throughout the day, including after 4pm.

During summer months (late March/early April to September), there may be some vitamin D production possible in the early part of the 4pm hour on clear days, but this window is brief and diminishes rapidly as the afternoon progresses. Research indicates that the majority of daily vitamin D production occurs when the sun is highest in the sky, typically around midday.

A practical rule of thumb is the "shadow rule": if your shadow is longer than your height, UVB levels are likely too low for significant vitamin D synthesis. After 4pm in the UK, shadows are typically long, suggesting reduced UVB availability. The Met Office UV Index can also provide guidance—higher UV Index values correlate with greater potential for vitamin D synthesis.

Therefore, relying on sun exposure after 4pm is generally not an effective strategy for maintaining adequate vitamin D levels, and alternative approaches should be considered, particularly during autumn and winter months.

Best Times for Vitamin D Synthesis in the UK

In the UK, the optimal window for vitamin D synthesis from sunlight is relatively narrow and varies considerably by season. The most effective period is between late March/early April and September, with the strongest UVB radiation typically occurring during midday hours, when the sun is at its highest point. During these months, exposing the skin to sunlight can contribute meaningfully to vitamin D status.

The British Association of Dermatologists and Cancer Research UK suggest that brief, frequent sun exposure to uncovered skin during the summer months may help maintain vitamin D levels. However, this must be balanced against skin cancer risk, and it's important not to burn. The amount of exposure needed varies significantly based on skin type, location within the UK, and the UV Index on a given day. Those with fair skin or a history of skin cancer should exercise particular caution and seek personalised advice.

From October through March, the sun's angle in the UK is too low for effective vitamin D synthesis, regardless of the time of day. During these months, even midday sun exposure produces negligible amounts of vitamin D. This period is often referred to as the "vitamin D winter," when vitamin D stores accumulated during summer months are gradually depleted. The NHS recommends that everyone in the UK should consider taking a daily vitamin D supplement containing 10 micrograms (400 IU) during autumn and winter.

Geographical location within the UK also matters—those living in Scotland and northern England experience even shorter periods of adequate UVB exposure compared to southern regions. Additionally, individuals with darker skin require longer sun exposure to produce equivalent amounts of vitamin D, as melanin absorbs UVB radiation. For these populations, supplementation becomes particularly important throughout the year, not just during winter months.

Factors That Affect Vitamin D Production

Multiple factors influence the skin's ability to synthesise vitamin D from sunlight, creating considerable individual variation in vitamin D status even among people with similar sun exposure patterns.

Skin pigmentation is one of the most significant factors. Melanin, the pigment responsible for skin colour, absorbs UVB radiation and competes with 7-dehydrocholesterol for available photons. Individuals with darker skin typically require longer sun exposure than those with fair skin to produce equivalent amounts of vitamin D. In the UK, people of African, African-Caribbean, and South Asian heritage are at particularly high risk of vitamin D deficiency and may benefit from year-round supplementation.

Age substantially affects vitamin D synthesis capacity. The concentration of 7-dehydrocholesterol in the skin decreases with age, and elderly individuals may produce significantly less vitamin D than younger people from the same sun exposure. This reduced synthetic capacity, combined with decreased outdoor activity and dietary intake, places older adults at increased risk of deficiency. The NHS and NICE recommend that adults over 65 should consider daily vitamin D supplementation throughout the year.

Body coverage and sunscreen use directly impact UVB exposure. Clothing blocks UVB radiation, and cultural or religious practices involving extensive body coverage can significantly limit vitamin D synthesis. In laboratory conditions, sunscreen with SPF 15 or higher, when applied thoroughly, can substantially reduce vitamin D production. However, in real-world settings, most people apply sunscreen inconsistently and in insufficient quantities, so some vitamin D synthesis typically still occurs. Sun protection remains the priority for skin cancer prevention, particularly for high-risk individuals.

Obesity is associated with lower circulating vitamin D levels, possibly because vitamin D is sequestered in adipose tissue, reducing its bioavailability. Individuals with a BMI over 30 may require higher doses of supplementation to achieve adequate vitamin D status. Additionally, certain medical conditions affecting fat absorption (such as Crohn's disease, coeliac disease, and cystic fibrosis) and medications (including some anticonvulsants, orlistat, and cholestyramine) can impair vitamin D metabolism. If you take regular medication, discuss vitamin D supplementation with your GP or pharmacist for personalised advice.

Alternative Sources of Vitamin D

Given the limited opportunity for vitamin D synthesis from sunlight in the UK, particularly during winter months and outside peak midday hours, alternative sources become essential for maintaining adequate vitamin D status.

Dietary sources provide vitamin D in two forms: vitamin D₃ (cholecalciferol) from animal sources and vitamin D₂ (ergocalciferol) from plant sources and fortified foods. Naturally rich dietary sources include:

• Oily fish such as salmon, mackerel, sardines, and herring (containing 5–25 micrograms per 100g portion)

• Egg yolks (approximately 1–2 micrograms per egg)

• Red meat and liver (modest amounts, typically 0.5–1 microgram per serving)

• Fortified foods including breakfast cereals, fat spreads, and some plant-based milk alternatives

However, it is challenging to obtain sufficient vitamin D from diet alone. A typical UK diet provides only 2–4 micrograms daily, well below the recommended 10 micrograms. Supplementation therefore plays a crucial role, particularly for at-risk groups.

The NHS recommends that certain populations should take a daily supplement containing 10 micrograms (400 IU) of vitamin D throughout the year, including:

• Infants from birth to one year (8.5–10 micrograms daily, unless consuming more than 500ml of infant formula per day)

• Children aged 1-4 years

• Pregnant and breastfeeding women

• People who are not often outdoors (housebound or in care homes)

• Those who cover their skin for cultural or religious reasons

• Individuals with darker skin

Additionally, everyone in the UK should consider supplementation during autumn and winter (October to March). Vitamin D supplements are available over the counter in various formulations, and vitamin D₃ is generally preferred over D₂ due to superior bioavailability in raising blood levels of 25(OH)D.

For those with confirmed deficiency, higher doses may be prescribed under medical supervision, typically following UK clinical guidelines. Treatment should be monitored, particularly in those with certain medical conditions.

If you experience symptoms potentially related to vitamin D deficiency—such as bone pain, muscle weakness, or fatigue—contact your GP. A blood test measuring 25(OH)D levels can determine your vitamin D status, and appropriate supplementation can be recommended based on the results. Vitamin D toxicity is rare but possible with excessive supplementation (typically above 100 micrograms or 4,000 IU daily over prolonged periods), so it is important to follow recommended doses unless under medical supervision.

If you suspect side effects from vitamin D supplements, report them through the MHRA Yellow Card scheme.

Frequently Asked Questions