Jack Dorsey Launches a Smart Sun Exposure Tracking App
If you've ever wondered how long you can safely stay in the sun or how much vitamin D you're actually getting, Jack Dorsey's new sun exposure tracking app, Sun Day, is designed to answer exactly that. Launched shortly after his minimalist chat app Bitchat, Sun Day is the Twitter co-founder’s latest foray into wellness technology. The app is now available through TestFlight on iOS and offers open-source access on GitHub for developers who want to experiment or expand on its functionality. At a time when tech users are more health-conscious than ever, Sun Day provides a useful and personalized way to monitor UV levels, prevent sunburns, and track vitamin D intake—factors increasingly linked to immunity, mood, and overall wellness.
Image Credits:Eva Marie Uzcategui/Bloomberg / Getty Images
How the Sun Day App Tracks Your UV Exposure and Vitamin D
What makes Sun Day more than just another weather or UV index app is its personalized approach. When you open the app, it shows the UV index in real-time based on your location, along with helpful environmental data like cloud coverage, sunrise, and sunset times. But what truly sets it apart is the ability to input your skin type (from six Fitzpatrick scale options) and clothing coverage. Based on this information, Sun Day calculates how long you can safely expose your skin before risking sun damage. Whether you’re light-skinned and burn easily or have more melanin protection, the app adapts its advice accordingly.
Once you’re ready, tap “Track UV exposure” to start a sun session. The app then tracks your exposure in real time, taking into account changing UV conditions. At the end of your session, it provides an estimate of your vitamin D synthesis—a number that varies depending on your skin type, the UV index, and time spent in direct sunlight. This is especially helpful for those trying to maintain healthy vitamin D levels without overexposing themselves to UV rays, which are known to cause premature skin aging and increase skin cancer risk.
Open Source Innovation Behind the Sun Exposure Tracking App
Unlike many health apps hidden behind subscriptions or paywalls, Jack Dorsey has opted for transparency and community contribution. The code for Sun Day is available on GitHub, allowing anyone to audit, fork, or modify it. For the coding community, this is a unique opportunity to build on a well-designed framework—especially as Dorsey avoided popular AI coding tools like Cursor and Claude Code in favor of a lesser-known open-source platform called Goose.
This move aligns with the “vibe-coding” culture where developers focus on intuitive, minimalist, and user-centric applications rather than bloated software. With Sun Day, the emphasis is clearly on simplicity and functionality. There’s no unnecessary friction—just accurate UV data, intuitive vitamin D tracking, and smart personalization. The open-source angle also invites health researchers and app developers to adapt the tech for medical use, especially in areas like dermatology, pediatrics, and preventive care.
Why Sun Day Matters in a Wellness-First Digital Era
As people become more aware of how sun exposure impacts long-term health, the demand for tools that help balance safe sun exposure and vitamin D intake is growing. For those who live in colder climates or work indoors, vitamin D deficiency is a common concern. Meanwhile, overexposure to UV rays remains a leading factor in skin damage and cancer. By offering personalized recommendations and tracking tools, Sun Day provides a practical solution for both challenges.
This new sun exposure tracking app isn’t just a niche gadget—it speaks to a broader cultural shift where tech is expected to be both empowering and health-conscious. Jack Dorsey’s track record of creating platforms with social impact (from Twitter to Square) adds credibility to this latest health-centric project. Whether you’re a tech enthusiast, a health-conscious user, or someone curious about how sunlight affects your well-being, Sun Day brings you one step closer to making informed decisions—without relying on guesswork or one-size-fits-all advice.
With the rise of biohacking, personalized wellness, and digital health tools, apps like Sun Day fill a vital gap between awareness and action. Dorsey’s entry into sun health tracking with a sun exposure tracking app reflects not just a personal coding experiment but a useful tool that leverages data and simplicity to promote healthier living. Whether you're optimizing your vitamin D intake, managing sensitive skin, or just staying safe under the sun, Sun Day offers a reliable, user-first experience rooted in both science and smart design.
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