Just launched Sun Day ☀️ — the free UV & vitamin D app I wish existed last summer

Hey makers 👋

Today I'm launching Sun Day on Product Hunt and I'd love your honest feedback.

Quick backstory. Last summer my girlfriend came home from a "quick 20 minutes at the park" with shoulders the color of a stop sign.

I opened three different weather apps, they all showed the UV index as a tiny number next to the temperature, with zero context.

No alert, no "you've had enough", nothing. I got annoyed enough to build the thing myself.

What Sun Day does

- ☀️ Real-time UV for your exact GPS location (powered by Open-Meteo)

- 🧬 Personal vitamin D tracker, adapts to your skin type, clothing and age, minute by minute

- 🔔 Smart alerts, "you've hit your daily dose, time to cover up". No spam, only when it matters

- 🌙 Bonus: moon phase, sunrise/sunset, hourly UV forecast

- 🔒 100% on-device. No account, no ads, no tracking.

Stack / stuff you might care about

Built with Expo SDK 55 + React Native 0.79, Zustand for state, Open-Meteo for the UV data, and a pure astronomical formula for moon phase (no third-party API). Vitamin D synthesis is a weighted formula I spent way too many evenings tuning, feedback on that part especially welcome.

Why free and ad-free

Sun Day is built under DIH LABS, a French non-profit (loi 1901). We don't monetize attention, the association funds the work.

That's also why I'm launching on PH instead of buying ads: I need real humans to tell me what's broken.

The ask

Not upvotes, I want the feedback. If you try it, I'd love to know:

1. Is the UV dial instantly readable or confusing?

2. Does the vitamin D number feel trustworthy or magic-box-y?

3. What's missing before you'd leave it on your home screen?

👉 https://www.producthunt.com/products/sun-day?utm_source=twitter&utm_medium=social

Thanks for reading, happy to return the favor on any launches in health, wellness or weather ☀️