The best advice on sunlight is also the simplest: get some. Get a lot more than you think you need. Don't overthink it.

That is the whole article for most people, and it is not wrong.

But if you want to know why it works, when it doesn't, and what determines whether a given hour outside is actually producing vitamin D — I'm here to help. What follows is the mechanism and what it implies for how you spend your time outdoors.

The Basics: How Your Skin Makes Vitamin D

Vitamin D synthesis starts with a cholesterol derivative in your skin called 7-dehydrocholesterol. When UVB strikes that molecule, it breaks a bond and rearranges into previtamin D3. Body heat finishes the conversion into cholecalciferol — vitamin D3 — which then moves through the liver and kidneys on its way to becoming the active hormone calcitriol.

The part worth underlining: only UVB does this. Not visible light, not infrared, not UVA. And UVB is a vanishingly small fraction of what the sun actually delivers — roughly 0.2 to 0.3 percent of the total solar spectrum reaching the surface on a clear day. The rest is about 43 percent visible light and 52 percent infrared, with UVA making up most of the remaining UV. The molecule your skin is waiting for lives in a narrow band between roughly 290 and 315 nanometers, and its availability at ground level changes drastically with the sun's angle.

The UV Index Threshold

You cannot make vitamin D when the UV index is below approximately 3.

The UV index is an aggregate measure of ultraviolet radiation reaching the surface, but it tracks UVB closely enough to use as a proxy. When the sun sits low in the sky, UVB has to travel through more atmosphere, and the ozone layer absorbs most of it before it reaches you. UVA, which has a longer wavelength, comes through regardless. You can stand outside on a cold clear January afternoon in Seattle, feel the sun on your face, and produce exactly zero vitamin D.

The implication is geographic and seasonal:

  • Above roughly 37°N latitude — San Francisco, Denver, Washington DC, and everything north of those — the UV index stays below 3 for a stretch of winter that runs anywhere from two to five months depending on how far north you go.
  • Below that latitude, in the subtropics and tropics, UV 3+ is reliably available year-round.

Check the UV index in your city →

Time of Day Matters

UVB intensity peaks when the sun is at its highest point in the sky. The rule of thumb is reliable: if your shadow is shorter than your height, UVB is available. If your shadow is longer, it largely isn't.

At mid-latitudes in summer, that window runs roughly from 10 AM to 3 PM. In winter, at the same latitudes, the window closes entirely — the sun never gets high enough.

Morning and evening sun, the golden hours, are dominated by red, infrared, and UVA wavelengths. They do other useful work — infrared drives mitochondrial function, UVA triggers nitric oxide release from the skin and lowers blood pressure — but they do not produce vitamin D.

The Window Glass Problem

Glass blocks UVB almost completely.

Sitting in a sunbeam at your desk feels warm because the longer infrared wavelengths pass through glass. You can get a tan or a burn from UVA through a car window. But the UVB portion of the spectrum is stopped cold by a few millimeters of silica. Every indoor "sunny spot" in your life produces no vitamin D at all.

If you want the synthesis, you need sky overhead — or at minimum an open window with unobstructed exposure.

How Much Is Enough?

The research converges on a blood 25-hydroxyvitamin D level of 40–60 ng/mL as the range where bone, immune, and metabolic markers look best. Call that the floor. My experience as a clinician is that people feel measurably better at the top of that range and above — the literature is describing adequacy, not optimum.

The standard prescription — 15 to 20 minutes of midday sun on arms and legs, three or four times a week — will get most fair-skinned people into the lower end of the range during UV-abundant months. It is a starting point, not a target. More surface area does more work. More frequency does more work. The sun is not a supplement with a daily ceiling; your skin regulates its own production and stops making previtamin D3 once local concentrations saturate.

Get your abdomen in the sun when you can. Large surface area, rarely exposed, densely vascularized, close to organs that use vitamin D at high rates. A shirt off for twenty minutes in the backyard does more than an hour of exposed forearms.

One thing worth knowing: as your skin tans, the time requirement goes up. A tan is not a free pass. It is your skin telling you that the same exposure now produces less vitamin D than it did two weeks ago. People who spend real time outside through the summer drift toward needing the longer exposures that darker-skinned people need year-round.

Calculate your personal vitamin D production →

How Skin Type Changes Everything

Melanin is a natural sunscreen. It's also a great deal more than that — a biological semiconductor, an antioxidant more powerful than vitamin C, a molecule that releases free electrons when struck by UV light. But for the purposes of vitamin D synthesis, melanin intercepts UVB before it reaches the 7-dehydrocholesterol deeper in the skin.

The effect is large. Dark skin can require four to six times longer to produce the same amount of vitamin D as fair skin under identical conditions:

  • Fitzpatrick Type I–II (very fair): 10–15 minutes of midday summer sun on arms and face produces roughly 1,000 IU.
  • Type III–IV (medium): 25–40 minutes for the same output.
  • Type V–VI (dark): 45–60+ minutes.

These are rough figures for midday sun at moderate latitudes in summer. They shift with UV index, exposed body surface, age, and individual biology.

The reason this matters epidemiologically: dark-skinned populations living at high latitudes are the most vitamin D–deficient demographic in the developed world. The skin that evolved for equatorial UV is doing exactly what it was supposed to do in an environment it wasn't designed for.

A Note on Supplementation

Vitamin D is not one molecule. It is a family of related compounds — D2 and D3 are the familiar ones, but the body also produces D4, D5, and a range of sulfated and hydroxylated metabolites that move through tissues in ways the literature is still mapping. The cholecalciferol in a softgel is one form. It is not the same thing as what your skin makes, and it is certainly not the whole system.

Dosing is a question for your doctor. Blood testing is inexpensive. The answer depends on your latitude, your baseline level, your skin, and what time of year you're asking.

Worth looking at, though: the traditional lifestyles of people who have lived at high latitudes for thousands of years. The Inuit, the Sami, the coastal Scandinavians, the Aleut. No one was running a vitamin D deficiency epidemic in those populations before the modern food system arrived. What they did have: heavy marine diets rich in fatty fish, seal, and organ meats that carry vitamin D directly. Cold exposure as a routine daily input. Long, dark, uninterrupted winter nights. Infrared from hearth fires and, later, saunas — wavelengths that do their own quiet work on mitochondrial function and tissue repair. Vitamin D was part of a system, not a standalone nutrient. The system included what they ate, how they slept, and what light they stood in front of during the months when the sun could not reach them.

The modern version of that is not a pill. It is a closer look at what the whole package was doing.

The Bigger Picture

Optimizing a single sunlight-derived molecule is a narrow frame on a broad phenomenon. The same midday exposure that produces vitamin D is also delivering red and infrared light to your mitochondria, UVA to your nitric oxide stores, and a full-spectrum signal to your suprachiasmatic nucleus that tells your body what time of day it is. Melanin stimulated by UV produces beta-endorphin and alpha-MSH downstream, which is part of why sun on skin feels the way it does.

If vitamin D is the only thing you're getting from the sun, you're leaving most of the benefit on the table. The people with the best vitamin D levels are usually also the people whose circadian rhythms, cardiovascular markers, and mood look better — not because vitamin D is doing all that work alone, but because the behavior that produces vitamin D produces the rest of it too.

Explore light protocols for your health goals →

This article reflects current photobiology research. Individual needs vary — a 25-hydroxyvitamin D blood test is inexpensive and tells you where you actually stand.