Jack Dorsey, the co-founder of Twitter and CEO of Block, has once again stepped into the wellness tech space. After launching his Bluetooth-based messaging app Bitchat, he’s now introduced a health-focused innovation — the Sun Day app. Built to help users track UV exposure and vitamin D synthesis, Sun Day combines science-backed calculations with clean UI and open-source ethics. But how exactly does it work — and why is it already creating buzz?
What Is the Sun Day App?
The Sun Day app is a minimal yet smart tool designed to help users understand how much sun exposure they need to meet their vitamin D intake goals — while staying safe from skin damage. It leverages location data, scientific models, and manual input to calculate both UV-related skin burn risk and vitamin D production potential.
Key Functions:
- Tracks UV Index at your current location.
- Estimates vitamin D synthesis from sunlight.
- It provides customization options for six different skin types.
- It adjusts exposure by taking into account the coverage of clothing.
- Displays cloud cover and sunrise/sunset times.
- Manual session tracking is required.
- The app syncs with Apple HealthKit to retrieve historical data.
How Does It Work?
When you open the app, you’re prompted to:
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- Select your skin type (from I to VI).
- Indicate your clothing level.
- Enable location access for UV Index data.
- Optionally, input your age and link Apple Health.
To start tracking, tap “Track UV Exposure”. The app starts a session timer. When you’re done, tap again to end the session.
Based on your skin’s melanin, UV strength, and session length, the app estimates:
- Time until sunburn (for safety)
- Vitamin D produced during the session
It also provides daily totals so you can monitor trends.
The Tech Behind the App: Goose, GitHub & Open Source
Sun Day is more than a wellness tracker — it’s part of Jack Dorsey’s broader open-source development philosophy.
- The app’s code is available on GitHub, meaning anyone can view, clone, or contribute.
- It was developed using Goose, an open-source coding tool built by Block (Dorsey’s company). Goose acts as an AI coding assistant, streamlining app development.
- Dorsey referred to this process as “vibe-coding” — building quickly and learning by doing.
This open structure has already attracted developers and early adopters through Apple’s TestFlight, where the app is currently in beta.
Features at a Glance
| Feature | Description |
|---|---|
| UV Index Tracking | Real-time UV levels based on location |
| Vitamin D Synthesis Estimator | Calculates conservative estimates from exposure |
| Manual Input Toggle | Requires starting/stopping sessions manually |
| Skin Type & Clothing Control | Adjusts calculations based on user skin type and clothing |
| Apple Health Integration | Syncs with HealthKit for historical vitamin D tracking |
| Open Source on GitHub | Community-editable codebase |
| Built with Goose | Coded using Block’s AI developer tool |
What’s Missing (for Now)?
While powerful, the app has limitations that Dorsey openly acknowledges in the white paper:
- The app doesn’t automatically detect light, instead relying on a manual toggle to turn it on and off.
- The app currently does not account for factors such as height, weight, or seasonal variations.
- No real-time blood vitamin D integration—though this may come in future versions.
- The app is currently only accessible through TestFlight and is not yet available on the App Store.
Why This Matters
In an era when screen time is up and outdoor time is down, the Sun Day app promotes a return to natural health — using tech responsibly. It aligns with growing interest in biohacking, personalized wellness, and preventive health through digital tools.
More importantly, it shows a new side of Jack Dorsey: a tech mogul experimenting with AI-assisted open-source coding for real-world health problems.
Final Thoughts
The Sun Day app might seem niche, but its implications are wide. From bio-individual sun exposure tracking to merging AI development with health science, Dorsey’s latest project is a fascinating step in tech-driven self-care.
As it evolves from beta to public release, Sun Day could become a go-to for health-conscious users looking to balance sun safety with vitamin D goals — all while maintaining control over their data and habits.