Vitamin D & the Sun

Ultraviolet radiation splits into UVA (315–400 nm) and UVB (280–315 nm). Only UVB in the narrow band of 290–315 nm carries enough energy to isomerise 7-dehydrocholesterol in the skin into pre-vitamin D3. UVA rays are longer-wavelength and lower-energy — they penetrate deeper (causing tanning and skin aging) but cannot trigger the photochemical reaction that starts vitamin D synthesis. This is also why tanning beds that emit mostly UVA do not produce vitamin D.

The angle of the sun above the horizon determines how much atmosphere UVB must travel through before reaching your skin. At low angles — early morning, late afternoon, or any time in winter above roughly 35° latitude — UVB is almost entirely absorbed by the ozone layer and never reaches the surface. The 'shadow rule' captures this well: if your shadow is longer than your height, UVB is too weak for vitamin D synthesis. In winter at northern latitudes, this condition persists all day for months. Below UVI 3, even prolonged exposure produces negligible amounts.

Standard window glass and car windshields block nearly all UVB while transmitting most UVA and visible light. This means you can get a tan sitting by a sunny window — UVA passes through — but you cannot synthesise vitamin D. Only special quartz glass, used in some medical UV devices, transmits UVB. The practical implication: that warm winter sun through your office window feels good but contributes nothing to your vitamin D levels.

The UV Index (UVI) is an international standard that measures the erythemally-weighted UV intensity at ground level. It runs from 0 (none) to 11+ (extreme). The app uses UVI ≥ 3 as the synthesis threshold because below that level UVB intensity is too low for meaningful vitamin D production, regardless of how long you stay outside. This threshold is consistent with the research of Holick and colleagues. The solar elevation needed to reach that UVI is not fixed: it depends on ozone, which varies with latitude and season, and ranges from about 29° to about 42°.

Not completely. Heavy overcast reduces UVB by 70–90%; thin or broken cloud cover by around 20–50%. Paradoxically, certain broken-cloud formations scatter UV and can briefly push ground-level UVB above clear-sky values. The app uses real UV data from Open-Meteo when available, so the minutes shown already account for current cloud conditions — no manual adjustment needed.

Yes. Vitamin D synthesis requires direct UVB exposure on bare skin. Clothing blocks most UVB, even thin fabrics. In the shade — under a tree or awning — diffuse UVB is minimal and insufficient for meaningful synthesis. Sunscreen SPF 30+ reduces synthesis by approximately 95%; the practical recommendation is to go without for the first 10–20 minutes (depending on your skin type) and apply it afterwards if you stay in the sun. Important: for other benefits such as nitric oxide release, circadian rhythm regulation, and serotonin production, direct sun is NOT required — being outdoors on a bright day already helps. But for vitamin D, bare skin exposed to direct sunlight is necessary.

Yes, but modestly. Body hair acts as a physical barrier to UVB radiation. Research by Galvez et al. (2015) quantified a 'Hair UV Protection Factor': sparse, fine body hair provides the equivalent of roughly SPF 1–3, while very dense, dark terminal hair can reach SPF 2–4+. A Monte Carlo simulation by Huang et al. (2020) showed that shaving the limbs increases UV transmission to follicular stem cells from roughly 5% to 20%, indicating significant attenuation by body hair — though this is measured at depth and does not translate directly into an equivalent UVB reduction at the skin surface. However, no clinical study has directly measured the impact of body hair on serum 25(OH)D levels. Major scientific reviews of vitamin D photosynthesis determinants — including Neville et al. (2021) — do not list body hair as a significant factor. In practice, the most heavily haired areas (chest, back) are usually covered by clothing anyway, and the effect on commonly exposed areas like forearms and lower legs is small (SPF 1–2 range). The app does not include a body hair adjustment because the effect is minor compared to skin type, UV index, body area exposed, and age — and is already partially captured by the body area parameter.

For vitamin D synthesis, in practice yes — as long as the UV index is similar in each segment. The skin integrates the total UVB dose following the Bunsen–Roscoe reciprocity law: the photoreaction depends on the product of intensity × time, not on how the time is split. In vivo studies confirm that equivalent rises in vitamin D3 are obtained with high irradiances over short periods or low irradiances over long periods. Three important caveats. First, the per-session ceiling: previtamin D3 reaches its cutaneous maximum at around one third of the Minimal Erythemal Dose (MED), and at 1 MED synthesis halts because UVB converts previtamin D3 into inert compounds (lumisterol and tachysterol). If a single long session exceeds that ceiling, extra minutes add no vitamin D, only burn risk — whereas ten 1-minute sessions never approach saturation. Second, the minimum UVI threshold: a measurable rise in serum 25-OH-D requires about 18–20 mJ/cm² of UVB, roughly UVI ≥ 3, so below that level no amount of accumulated minutes produces meaningful vitamin D. Third, thermal kinetics are slow: the conversion of previtamin D3 to vitamin D3 has a half-life of ~2.5 h at skin temperature and completes in ~8 h, so the vitamin D you generate during the day continues forming after exposure ends. Practical takeaway: repeated suberythemal exposures produce the same vitamin D with less DNA damage than a single long session — which is exactly the pattern medical guidelines (Holick 2007; Neville et al. review, JBMR Plus 2021) recommend.

For the same amount of vitamin D, exposing more skin for less time is almost always better — more efficient and less harmful. The reason is that vitamin D production per patch of skin saturates: previtamin D3 photodegrades to inert lumisterol and tachysterol, so each area of skin has a ceiling no matter how long you stay out. That ceiling scales with exposed area, not with how tanned you are — roughly ~19,000 IU for the whole body at 1 MED, so ~1,900 IU for 10% of the body (face + hands) versus ~7,700 IU for 40% (swimsuit). With very little skin exposed you may be physically unable to reach a 2,000 IU target in a single session, whereas with more skin you get there in a few sub-erythemal minutes. The health argument follows the same logic: vitamin D per minute on an already-dosed patch falls (saturation), but DNA/erythema damage per minute stays roughly constant, so the vitamin-D-to-damage ratio is best at low per-patch doses. Sunburn depends on the local dose versus your Minimal Erythemal Dose (MED), independent of area — concentrating a dose on a small patch pushes it toward or past 1 MED (a burn), while spreading the same total over more skin keeps every patch well below its threshold (no burn). That is exactly why guidelines recommend ~¼–½ MED over a large body area, a few times a week. Two extras favour more skin: exposed skin releases nitric oxide under UVA that lowers blood pressure, and this scales with area, not with vitamin D; and concentrating exposure on one patch tans it, raising its MED and making it progressively less efficient over repeated sessions. The only case for 'little skin, long time' is practical — when clothing or context won't let you uncover more — and even then you'll often fail to reach the target in one go.

A full-body exposure to 1 MED (Minimal Erythemal Dose — just before any reddening) produces around 10,000–25,000 IU of vitamin D. A standard supplement capsule contains 400–2,000 IU. A 15–20 minute midday session in summer with arms and legs exposed can easily exceed a week of typical supplementation. However, the sun has a built-in safety mechanism (photodegradation) that prevents overdose — a protection oral supplements lack.

No. Solar synthesis is self-limiting. Once pre-vitamin D3 accumulates in the skin, continued UVB converts it into inert compounds — lumisterol and tachysterol — rather than allowing further build-up. This photodegradation ceiling means your body never produces toxic levels from sun exposure, no matter how long you stay outside. This is why the IOM Tolerable Upper Intake Level of 4,000 IU/day applies specifically to oral supplementation, not to sunlight.

Both have real advantages. Sun-produced D3 is bound to skin proteins and released slowly into the bloodstream, giving it a longer half-life than oral D3. Sun exposure also provides UVA-driven nitric oxide release, circadian light cues, and other benefits no pill can replicate. Oral D3, on the other hand, is controllable, dose-precise, and available year-round. The practical answer: use the sun when conditions allow, supplement when they don't — they complement each other.

D3 (cholecalciferol) is significantly more effective than D2 (ergocalciferol) at raising and maintaining 25-OH-D blood levels — research suggests D3 is roughly 2–3× more potent per IU. D3 is also the form your skin produces. D2, derived from plant and fungal sources, is used in vegan supplements but underperforms D3 at equivalent doses. Look for 'cholecalciferol' on the label; vegan D3 from lichen is also available.

Standard reference ranges for serum 25-hydroxyvitamin D: below 20 ng/mL (50 nmol/L) is deficient; 20–30 ng/mL is insufficient; 30–50 ng/mL is sufficient. The Endocrine Society considers 40–60 ng/mL optimal for non-skeletal benefits. Above 100 ng/mL (250 nmol/L) carries toxicity risk from supplementation. Most people in northern latitudes test below 20 ng/mL in winter without supplementation — a widespread but easily correctable deficiency.

Choose D3 (cholecalciferol) unless you're vegan, in which case lichen-derived vegan D3 is available. For most adults: 1,000–2,000 IU/day if you get regular sun; 2,000–4,000 IU/day if you have limited sun exposure. The Endocrine Society's upper intake limit is 4,000 IU/day for adults from supplements. Higher doses for correcting deficiency exist but should be guided by blood tests. Testing 25-OH-D before and after 3 months of supplementation is the best way to find your personal effective dose.

Vitamin D3 increases calcium absorption from the gut. Vitamin K2 — particularly the MK-7 form — activates two proteins: osteocalcin, which helps fix calcium into bone, and matrix Gla protein, involved in regulating calcium in soft tissues. In observational studies, higher menaquinone (K2) intake has been associated with less arterial calcification, though intervention evidence is still limited. Natural K2 sources include natto, aged hard cheeses, and egg yolks. This is why many supplement protocols pair D3 with 100–200 mcg of MK-7, especially at higher D3 doses.

Magnesium is a cofactor for both enzymes that convert vitamin D to its active forms (25-OH-D and then 1,25-OH-D). Without sufficient magnesium, supplemental D3 may not convert properly, and supplementation can push magnesium levels lower, worsening any existing deficiency. An estimated 50% of Western populations are magnesium-insufficient. Good dietary sources: pumpkin seeds, dark chocolate, spinach, almonds, avocado. Magnesium glycinate or malate supplements are well tolerated if dietary intake is low.

Vitamin D is fat-soluble. It requires dietary fat in the gut to form micelles — tiny fat-protein packages that carry the vitamin through the intestinal wall into the bloodstream. Studies show that taking D3 with a fat-containing meal increases absorption by 30–50% compared to taking it fasted. The practical rule: take it with your fattiest meal of the day. A tablespoon of olive oil, a handful of nuts, or a meal with salmon all provide sufficient fat. This applies equally to vitamins A, E, and K2.

Very few foods contain meaningful amounts: fatty fish (wild salmon 600–1,000 IU per 100 g; mackerel, sardines, herring), cod liver oil (~400 IU per teaspoon, also rich in vitamin A), egg yolks (~40 IU each, more if the hen had UV exposure), UV-exposed mushrooms (can provide 400+ IU per 100 g when dried gills-up in sunlight), and beef liver (small amounts). Fortified foods (milk, cereals, plant milks) add modest amounts depending on country. Diet alone realistically cannot reach the 1,000–4,000 IU range without supplementation.

With your largest meal of the day — primarily for the fat absorption benefit. Morning or midday (with breakfast or lunch) is generally preferred over evening because vitamin D may mildly increase alertness and some evidence suggests evening supplementation could slightly interfere with melatonin production. That said, consistency matters far more than timing: take it at whatever point in your day you will reliably remember.

Nitric oxide (NO) is a signalling molecule that dilates blood vessels, lowers blood pressure, and improves circulation. The skin stores large quantities of nitrite and nitrosothiol compounds that release NO into the bloodstream within minutes of UVA exposure. This is entirely independent of vitamin D synthesis — it happens through glass, in partial shade, and even in winter when UVB is absent. Research by Richard Weller's group at the University of Edinburgh suggests that UVA-driven NO release may explain the well-documented association between sun exposure and reduced cardiovascular mortality, beyond what vitamin D alone can account for.

Light is the primary zeitgeber — 'time-giver' — for the human circadian clock. Morning sunlight rich in short-wavelength blue light (460–480 nm) signals the retinal ganglion cells, which in turn tell the suprachiasmatic nucleus (SCN) in the brain to suppress melatonin and trigger the cortisol awakening response. This anchors the 24-hour body clock. Insufficient morning light exposure delays sleep onset, disrupts hormone timing, and is associated with mood disorders and metabolic dysfunction. The ideal window is the first 30–60 minutes after waking, outdoors — even on overcast days, outdoor light is typically 10–50× brighter than indoor lighting.

Dawn light has a completely different spectral profile from midday sun: it is rich in red and near-infrared wavelengths (600–1,000 nm), with little UVB and moderate blue light. This infrared light penetrates several centimetres into tissue and activates cytochrome c oxidase in mitochondria, improving ATP production and cellular energy. It also clears overnight melatonin more gently than high-intensity blue light, easing the transition to wakefulness. Red-light and near-infrared therapy devices (wavelengths 630–850 nm) replicate part of this spectrum for therapeutic use.

Yes. UV exposure independently modulates immune function through mechanisms separate from vitamin D: it induces regulatory T cells and alters antigen-presenting cells in ways that suppress inflammatory responses. This is the basis for UV phototherapy used clinically in psoriasis, eczema, and vitiligo. Some researchers link this broader UV immunomodulation to the latitude gradient in autoimmune disease prevalence — multiple sclerosis, type 1 diabetes, and inflammatory bowel disease are all significantly more common at higher latitudes, a pattern only partially explained by vitamin D levels.

Strongly. Bright light stimulates serotonin synthesis via the raphe nuclei in the brainstem — this mechanism underpins light therapy as a first-line treatment for Seasonal Affective Disorder (SAD). Serotonin is also the precursor to melatonin: adequate daytime serotonin supports better melatonin production at night, improving sleep quality. Separately, high-lux outdoor light triggers dopamine release in the retina, which appears to slow myopia progression in children. Population studies consistently show associations between time outdoors, mood, and sleep quality that extend well beyond vitamin D status.

Because Earth's axis is tilted about 23.4° relative to its orbit. As we travel around the Sun, each hemisphere spends half the year tilted toward it (days getting longer) and half tilted away (days getting shorter). The effect grows with latitude: at the equator day length barely changes by minutes, in Madrid it swings between roughly 9 and 15 hours, and above the polar circle it reaches the extremes of midnight sun and polar night. At the March and September equinoxes the day lasts ~12 hours everywhere on Earth. The monthly table on every city page and the Explore tab let you see this cycle for any place and any day.

It's the window when the sun sits less than ~6° above the horizon: roughly the last hour before sunset and the first after sunrise. With the sun that low, light crosses far more atmosphere and Rayleigh scattering strips out most of the blue, letting the reds and oranges through, with long soft shadows. It's the light photographers and filmmakers love, and in summer a comfortable slot for a walk or outdoor exercise, since heat and radiation are far lower than at midday. The app shows each day's golden hour in the sun panel.

Practically no. Synthesis needs UVB, and UVB is precisely the part of the spectrum that gets filtered most when the sun is low: the light's path through the atmosphere and the ozone layer grows so long that almost no UVB reaches the skin. That's why the app computes the synthesis window from solar elevation rather than daylight hours: a day can have 15 hours of sun yet only 6–8 useful for vitamin D. Golden hour still delivers the rest: the light signal for your circadian rhythm, mood, and the red and near-infrared wavelengths from the questions above.

Above the polar circles (from ~66.6° latitude), Earth's tilt means the midsummer sun never sets (midnight sun) and the midwinter sun never rises (polar night). The closer to the pole, the longer it lasts: in Tromsø or Svalbard it spans whole weeks or months. The app flags it in the sun panel when it happens, and through the long polar winter vitamin D can only come from diet or supplementation.