SolGlow: Vitamin D Tracker

SolGlow turns your Apple Watch's Time in Daylight into a real estimate of the vitamin D you produced from sunlight today, and tells you exactly how much sun to aim for tomorrow.

It's the vitamin D companion that respects your data. SolGlow does its work on your iPhone — your profile, your food log, your location history, your Apple Health data, all of it stays on your device. The only network call SolGlow makes is a UV forecast lookup with no account, identifier, or device fingerprint attached.

PLANNING

- Weekly Sun Plan — a recommended number of minutes outside each day of the upcoming week, balancing vitamin D need against burn safety. The plan adapts to your serum estimate, your skin type, and the day's forecasted UV.
- Annual Plan — a year-at-a-glance view of when UVB is strong enough at your latitude to produce vitamin D from sun, color-coded Insufficient → Marginal → Adequate → Excellent month by month.
- Plan adherence on the Dashboard — a planned-vs-actual progress bar that shows how close you stayed to your week's plan.

THE FUNDAMENTALS

- Reads your Apple Watch's Time in Daylight from Apple Health and combines it with the sun's elevation at your latitude, your skin type, and what you wore outside, to estimate today's cutaneous vitamin D synthesis.
- Folds in your daily supplement and dietary intake — log a salmon fillet, a glass of fortified milk, a 2,000 IU softgel, anything — and SolGlow keeps a running picture of where you stand.
- Surfaces a Deficiency Risk score that adjusts to your latitude, recent sun exposure, supplements, and skin type, with a clear breakdown of every factor.
- Warns when forecasted UV combined with the time you've been outside crosses the burn threshold for your Fitzpatrick skin type — Type I through VI all dialed in.
- Writes the estimated vitamin D back into Apple Health as Dietary Vitamin D so it shows up alongside your other nutrition data.

ALERTS WHEN YOU NEED THEM

Opt in to two local notifications, each fires at most once per day:
- Sunburn limit reached — when today's combined UV + outdoor time crosses your skin's threshold.
- Daily vitamin D goal met — when today's estimate hits your target IU.

DAILY DETAIL

Drill into any day in History to see hourly vitamin D production, hourly sunlight exposure, peak production time, and the day's outfit and skin-type assumptions.

HOME SCREEN WIDGET

Today's IU, today's sunlight minutes, and your daily-goal progress, on your Home Screen and Lock Screen. Refreshes whenever you open the app or your Apple Health data updates.

PRIVACY THAT IS REAL, NOT MARKETED

- No account. No login. No sign-up.
- No third-party analytics. No advertising. No tracking SDKs.
- Apple Health data is read only on the device.
- Location is used on the device for solar math, and sent off the device only as latitude and longitude when fetching a UV forecast from Open-Meteo. No identifier ever accompanies that request.
- Location history is throttled to roughly one fix every two hours and pruned automatically after 60 days.
- Your data is never sold. Ever.

REQUIREMENTS AND NOTES

- iPhone running iOS 17.2 or later.
- Time in Daylight is collected by Apple Watch (watchOS 9 or later). SolGlow works without an Apple Watch but its estimates become much less accurate.
- SolGlow is an information and estimation tool, not a medical device. It does not diagnose, treat, cure, or prevent any condition. Always consult a qualified healthcare professional before making decisions about sun exposure, supplements, or your health.