Jack Dorsey's New Sun Day App Helps Users Track Sun Exposure and Vitamin D

Photo credit: Eva Marie Uzcategui/Bloomberg / Getty Images

Jack Dorsey Unveils Sun Day: A Simpler Way to Monitor UV Exposure

Jack Dorsey, Twitter’s co-founder, continues his streak of launching new consumer apps. Shortly after introducing a messaging tool named Bitchat, Dorsey has released another experimental app called Sun Day. Designed for iOS and accessible through TestFlight, Sun Day aims to simplify how individuals track UV exposure and vitamin D intake. The app’s source code is open and available on GitHub, encouraging community involvement and transparency.

Dorsey shared the news on his social media, noting that Sun Day is designed to help users easily monitor their daily sun exposure and vitamin D levels. Unlike many contemporary developers using AI-powered coding tools like Cursor or Claude Code, Dorsey opted to build his app with Goose, an open-source coding platform.

How Sun Day Works

Upon opening Sun Day, users are provided with local UV Index information, weather details, as well as sunrise and sunset times relevant to their area. A major feature allows users to select their skin type from six options and specify their clothing coverage for the session. By combining these personal attributes with environmental factors, the app estimates how much sun exposure one can safely tolerate before risking a sunburn.

Image Credit: TechCrunch

To encourage healthy habits, Sun Day also displays the minimum recommended daily vitamin D intake. Tapping the “Track UV Exposure” button lets users start a monitoring session. Once completed, the app reports both the UV exposure and the amount of vitamin D synthesized during the session and across the day.

Deep Founder Analysis

Why it matters

The introduction of Sun Day highlights a growing trend of health-focused personal apps that leverage real-time environmental and user-specific data. For startups, this move signals a shift towards more personalized, proactive health management—particularly those that bridge the gap between wellness and actionable daily behavior. The openness of the Sun Day codebase also encourages a community-driven approach, inviting collaboration, innovation, and transparency—values increasingly prized in today’s tech ecosystem.

Risks & opportunities

As interest in digital wellness tools grows, reliance on accurate environmental data and personalization becomes both a risk and an opportunity. Market risks can include data accuracy, privacy concerns, and ensuring users act on recommendations responsibly. However, the opportunity to differentiate through meaningful user experiences, add-ons like integration with wearables, or partnerships with dermatology clinics could help startups stake their claim in a rapidly crowding space. Past examples show that apps rooted in public health or preventative care can gain traction quickly—think MyFitnessPal for fitness or Clue for women’s health—if executed well.

Startup idea or application

A founder might build on Sun Day’s premise by developing a holistic "Sun Health" platform. This could aggregate UV exposure, vitamin D tracking, and early skin anomaly detection (using AI photo analysis), with automated reminders based on location or outdoor plans. Integration with smartwatches and real-time weather APIs could create a powerful prevention and education tool, targeting both individuals and at-risk groups like outdoor workers.

Related DeepFounder Reading

• Meta Acquires Play AI: A Strategic Move in AI Voice Technology
• Windsurf Leadership Joins Google as OpenAI Acquisition Collapses: What It Means for Startups
• GenAI Set to Transform Prime Day Shopping: Insights for Startups

UV Tracking Consumer Health Apps Jack Dorsey Vitamin D Open Source

Visit Deep Founder to learn how to start your own startup, validate your idea, and build it from scratch.

📚 Read more articles in our Deep Founder blog.