How Much Sun for Vitamin D? The Surprise Is - It Isn't About Strength

Lets start by killing the first little myth: a few minutes of sun on your face and hands tops up your vitamin D for the day. It doesn't. 

When we worry about making enough vitamin D, we fixate on the strength of the sun — the season, the time of day, the clouds. All real. But there's a second number that quietly decides how much you make, and it's the one you can actually change: how much skin is catching the light. Get that wrong and the strongest July sun still leaves you short.

Your skin is the factory -  sunlight just flips the switch

First, another myth needs to be killed: vitamin D doesn't come from the sun, and it isn't really a vitamin.

Sunlight doesn't deliver vitamin D the way food delivers iron, instead UVB light hits a molecule in your skin and kick-starts a reaction. Your body finishes the job, then uses the result like a hormone — which technically makes it a prohormone, not a vitamin at all. The sun is just the trigger. Your skin is the factory.

More skin beats - more time

This is the bit the myth skips. Your vitamin D output rides on how much skin you expose as parts of the body gets saturated after a while.

The research is refreshingly blunt about it. In studies  different parts of the body was exposed to the same dose of UVB. People who bared their whole body made far more vitamin D than people who exposed only their upper body. 
Here's is the lovely twist: The group of bare people needed much less sun power and much less time to create enough vitamin D. Low, won't-burn-you doses are actually more efficient at making vitamin D than near-sunburn ones.
And once a patch of skin has made its quota for the day, it clocks off — so standing in the sun longer with one arm out does nothing. Show more skin all over for less time, and you win on both counts. 

Does hair block vitamin D? Ask a cow!

Your scalp is skin. Same machinery, same ability to make vitamin D, except for the hair growing out of your scalp blocks the very UVB the skin beneath it needs.  

In a 2025 study, dairy cattle were split into shaved and unshaved groups and put under UVB light. Despite getting less than half the dose, the shaved cows ended up with higher vitamin D than the unshaved ones. Because hair reduces how much UVB reaches the skin. Less light, more vitamin D — purely because their skin was bare. The coat, not the sunshine, was the bottleneck. 

Can you get vitamin D in the shade?

Shade does more than you'd expect. The shorter UVB wavelengths scatter across the open sky, so open shade still carries enough to make vitamin D - with far less of the UVA that ages skin. Sitting in light shade with plenty of skin bare can genuinely beat baking in full sun with very little showing. Maximise the area, ease off the intensity.

But remember: UV is still UV - burning and skin cancer are real, so this isn't a free pass to sit out for hours. And no - you can't get vitamin D through a window: glass blocks UVB, so sunlight indoors makes almost none. 

Your hair wants - the opposite

Here's the catch for your hair. Shade is kinder to your skin than to your strands, because it trims the UVB but lets relatively more UVA slip through.

UVA is what drives the free radicals that fade your colour and damages your hair. So shade-sitting is a great deal for your vitamin D and skin but not such a good deal for your lengths. Keep them covered.
The scalp underneath needs looking after too, but for the opposite reason: heat.
Warm weather thins your sebum so it spreads and mingles with sweat, and once that film is trapped under a hat, it turns into a cosy habitat for microbes.

We'd reach for this regimen

A summer scalp needs its own rhythm — starting with the scalp, not the strand.

Cleanse first: 008 Clarifying Wash Pro  dissolves build-up and resets a greasy, congested scalp. On tight, reactive days, 010 Slip Wash Pro cleanses gently with high slip and prebiotic support.
Then rehydrate — surface oil isn't hydration with 022 Hydro Mask Pro | Light for finer hair or 018 Hydro Mask Pro | Riche or coarser textures.
And between washes, 301 Deobiotic Citrus Mist  keeps things fresh and tackles odour at the source, so your hat stops being the enemy.

After summer: test, don't guess

You can't feel your vitamin D level, and you can't read it off how sunny your summer felt. Sunny memories are unfortunately not persistent and guessing takes you nowhere. 

A simple blood test answers three things guessing can't. Whether you're actually low, the only thing that justifies supplementing at all. Which form fits you — D3 generally raises your level more reliably than D2, and vegans can now get D3 from lichen. And how much, since the right dose is genuinely personal; people respond very differently to the same dose. 

Best time to check? As the dark months arrive, when your skin's own production drops toward nothing and a low level has nowhere to hide. Worth it especially if you're prone to shedding — your hair follicles rely on vitamin D to switch into their growth phase. Get the number, match the dose to it, and keep your bones, muscles, immune system and scalp covered the rest of the year.

Knowledge Is Power.

Blog tip: For the UV that does reach your scalp, see scalp sun protection and UV damage. For vitamin D's role in shedding, read Vitamin D – A Hidden Hero in Hair Loss?)

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NOTE!
The science here draws on peer-reviewed research on vitamin D synthesis, the vitamin D receptor in hair-follicle cycling, and skin biology. Supplementation guidance is general information, not medical advice. Please check your own levels and speak with a qualified practitioner before changing a supplement.