While most tech entrepreneurs pivot from social media to fintech or AI ventures, Jack Dorsey—co-founder of Twitter and CEO of Block—has ventured into an altogether more elemental pursuit: helping users optimize their vitamin D synthesis through precise UV exposure tracking. His latest creation, *Sun Day*, represents perhaps the most scientifically rigorous attempt yet to quantify something humans have done intuitively for millennia: determining how long to stay outside without burning.
The app, currently in beta on Apple’s TestFlight platform, requires users to manually toggle sessions—a deliberate design choice that prioritizes accuracy over convenience. Rather than relying on passive sensors (which might register false positives while sitting near windows), the system demands conscious user engagement. Users input personal variables including skin type, age, and clothing coverage, while the app pulls location data to calculate local UV indices and daylight patterns.
What distinguishes *Sun Day* from typical wellness apps is its multifactor scientific modeling approach. The system combines environmental UV data with physiological inputs to estimate both burn risk and vitamin D production—calculations that consider variables ranging from cloud cover to melanin concentration. Integration with Apple HealthKit enables historical analysis, while future updates promise incorporation of height, weight, and blood test results for enhanced personalization.
The privacy implications merit consideration: location tracking remains essential for UV index calculations, though Dorsey’s team has open-sourced the code on GitHub, enabling public scrutiny of data handling practices. The app employs conservative estimation algorithms, prioritizing safety over optimization—a revitalizing approach in an industry often criticized for encouraging addictive behaviors.
Perhaps most intriguingly, *Sun Day* was developed using Goose, an AI-assisted coding tool created by Dorsey’s team. This meta-narrative—AI helping humans better understand their relationship with natural light—captures the paradoxical nature of modern wellness technology. The app follows Dorsey’s recent app-building initiatives, which previously included the release of Bitchat. Beyond its technological innovation, *Sun Day* serves as a preventive health tool designed to address the challenges of modern indoor lifestyles.
The app targets individuals with vitamin D deficiency risk factors: those with limited outdoor exposure, darker skin tones, or residence at high latitudes. Whether manual session logging will achieve sufficient user compliance remains the critical question determining *Sun Day’s* long-term viability in the increasingly crowded digital health ecosystem. As blockchain technology continues to provide new environments for machine learning applications, Dorsey’s latest venture demonstrates the ongoing convergence of AI and everyday wellness solutions.
