Vitamin D is unusual among nutrients: most of what your body uses doesn't come from food at all. It comes from your skin, manufactured on demand when ultraviolet light hits it. That makes it genuinely hard to track. You can count the IUs in a capsule, but the far larger and more variable source — the sun — is invisible. SolGlow exists to make that invisible source visible, estimating the vitamin D your skin produced today from the data your iPhone and Apple Watch already gather.
This article explains the physiology behind that estimate: how sunlight becomes vitamin D, which factors swing the result by an order of magnitude, and how SolGlow combines them — entirely on your device.
How sunlight becomes vitamin D
The reaction happens in your skin. Specifically, UVB radiation (ultraviolet light in the 290–315 nm band) strikes a cholesterol-derived molecule called 7-dehydrocholesterol in the skin and converts it to previtamin D₃, which then becomes vitamin D₃. Your liver and kidneys finish the job, turning it into the active hormone your body uses for bone health, immune function, and more.
The critical detail is that only UVB drives this reaction — not UVA, not visible light, not warmth. And UVB is the part of the spectrum most aggressively filtered by the atmosphere. That single fact explains why so much about vitamin D synthesis comes down to geometry: where the sun is in the sky.
The factors that change everything
Two people standing in the same sunshine can synthesise wildly different amounts of vitamin D. SolGlow's model accounts for the variables that matter most:
1. The sun's angle (latitude, season, time of day)
When the sun is low, UVB takes a longer, more oblique path through the atmosphere and much of it is absorbed before reaching the ground. This is why winter sun at high latitudes can produce almost no vitamin D even on a bright, cloudless day — a phenomenon often called "vitamin D winter." It's also why midday sun is dramatically more productive than early-morning or late-afternoon sun. SolGlow uses your location to compute the sun's elevation and the resulting UVB availability throughout the day.
2. Skin type
Melanin is a natural sunscreen. More melanin protects against burning but also slows vitamin D synthesis, so darker skin generally needs longer sun exposure to make the same amount of vitamin D as lighter skin. SolGlow uses the Fitzpatrick skin-type scale (I–VI) you set in your profile to scale the estimate — and to tune its burn-risk warnings, because the two are in tension and need to be balanced honestly.
3. How much skin is exposed
Vitamin D synthesis scales with the surface area of skin receiving UVB. Shorts and a T-shirt expose far more skin than a coat and trousers. SolGlow lets you describe your typical outdoor outfit (and even a weekday clothing schedule) so the estimate reflects how much of you the sun could actually reach.
4. Time actually spent in daylight
This is the input that used to be guesswork — and it's where the Apple Watch changes the game. Recent Apple Watches record Time in Daylight, a measurement of how long you've spent in bright outdoor light, written straight to Apple Health. SolGlow reads it directly, so instead of asking you to estimate "how long were you outside?", it uses a real, sensor-backed figure.
Why UV index alone isn't enough. A high UV index tells you the sun is intense, but not how long you were in it, how much skin was exposed, or how your skin type responds. SolGlow combines the forecast UV with your daylight minutes, outfit, and skin type — which is why its estimate is personal rather than generic.
Adding food and supplements
Sunlight is the biggest lever, but it isn't the only source. SolGlow lets you log dietary vitamin D — from fatty fish, fortified foods, egg yolks — and any supplements you take, then combines all three streams into a single daily total measured against your goal. On a grey winter day, that supplement might be most of your intake; on a sunny summer afternoon outdoors, the sun could put you well over target on its own. Seeing them together is the point.
Balancing benefit against burn risk
There's an inherent tension in sun exposure: the same UVB that makes vitamin D also causes sunburn and, over time, skin damage. A responsible vitamin D tool can't only chase the upside. SolGlow pairs its estimate with an optional burn-risk warning that watches the forecast UV against your time outside and your skin type's threshold, and flags when you're approaching the point where the costs start to outweigh the benefits. The goal is enough sun, not maximum sun.
The connection to stress and cortisol
Morning light does something beyond vitamin D: it helps set your circadian rhythm and sharpens the Cortisol Awakening Response, the healthy morning cortisol spike that supports steady energy and mood through the day. This is why SolGlow connects to its companion app, CalmTide, our cortisol and HPA-axis tracker. With both installed, SolGlow passes your morning-sun minutes and a CAR score to CalmTide through a secure on-device link, where they feed your weekly stress-balance score. Two apps, one picture: the sun's role in both your vitamin D and your stress recovery. You can read more in our deep dive on the science behind CalmTide.
Why it all runs on your device
Everything above — the solar geometry, the skin-type scaling, the daylight minutes, the food log — is computed locally on your iPhone. SolGlow has no account and no analytics; the only thing it sends over the network is an anonymous UV-forecast lookup for your area. Your health data, location history, and vitamin D estimates never leave your device. Privacy isn't a feature bolted on afterward; it's a constraint the whole design is built around.
What the estimate is — and isn't
SolGlow produces a well-reasoned estimate, not a blood test. Individual vitamin D synthesis varies with age, body composition, genetics, and skin factors no app can fully capture, and the only way to know your actual vitamin D status is a blood test ordered by a clinician. What SolGlow gives you is something a blood test can't: a daily, personalised sense of whether your habits — sun, food, and supplements together — are trending toward enough or falling short, so you can adjust before a deficiency sets in. If you have concerns about your vitamin D levels, talk to a healthcare professional.
Track the sun, not just the supplement
SolGlow turns Apple Watch Time in Daylight, your skin type, and your location into a real estimate of the vitamin D you produced today. Free, private, on the App Store.
Explore SolGlowRelated reading: How much sun do you actually need for vitamin D? — why the "15 minutes a day" rule falls apart, and how to find your own number.
This article is for general education and is not medical advice. SolGlow provides vitamin D estimates based on published physiology and is not a medical device. Consult a healthcare professional about your vitamin D status or sun exposure.