Frequently Asked Questions

OPSIN helps you get the right light at the right time. It estimates your daily vitamin D from the sun (plus any supplements you log), tracks your circadian light exposure (bright mornings, dim evenings), tells you the best times to go outside today, and warns you before you'd get too much sun for your skin type.

Most apps optimize a single number. OPSIN models your whole light day from one consistent engine — so your "safe sun limit" and your "vitamin D goal" are always calculated from the same physics and never contradict each other. It combines vitamin D, circadian rhythm, and burn safety into one picture, with a plan for when to go outside.

Core features are free. A Premium subscription unlocks the full recommendation engine, all history and insights, unlimited light readings, the AI coach, and tracking tools. You can cancel anytime; subscriptions are billed through your app store.

They are modeled estimates, not measurements. We use published photobiology (UV Index, your skin type, exposed skin, age, and the sun's angle) to estimate how much vitamin D your skin can make — and we cap it at the point where skin physically stops making more (the "plateau"). Real synthesis varies a lot between individuals (±30–50%), so treat the numbers as a helpful guide, and confirm your actual status with a blood test (25-OH-D) if it matters to you.

This is exactly the kind of contradiction OPSIN is designed to avoid. Your skin stops making vitamin D at roughly the same exposure where sunburn risk begins, so on most sunny days "you've optimized your vitamin D" and "you've reached your safe limit" arrive together. On weak-UV days (winter, high latitude, heavy cloud) the sun physically can't reach your target — OPSIN will tell you that honestly and suggest topping up with a supplement rather than sending you out to burn.

From a public forecast service (Open-Meteo). The UV Index we use already accounts for cloud cover, so a cloudy day shows lower UV than a clear one. UV also depends on your latitude, the season, time of day, and elevation.

It estimates it. On phones, the camera's own exposure settings (shutter, ISO, aperture) encode how bright the scene is — that's what real light-meter apps read, and it's far more reliable than reading pixel brightness. Expect roughly ±10% after you calibrate it once against a known reference; it's meant as a directional signal, not a laboratory instrument.

Vitamin D score — how close you are to your daily vitamin D target (sun + supplements + diet). Circadian score — whether you got bright light early and dim light late, weighted for its effect on your body clock. OPSIN score — the overall quality of your light day, combining both plus indoor-light adequacy and your daylight time from Apple Health.

It estimates your time-to-burn for your skin type and current UV, and warns you as you approach it — but it can't see your actual skin, sweat, sunscreen reapplication, or reflected UV from snow/water/sand. Always use your own judgment and standard sun protection. If you have a history of skin cancer or photosensitivity, follow your doctor's advice, not an app.

That's a decision for you and your healthcare provider. OPSIN lets you log supplements so your daily total reflects everything you take, but it does not prescribe doses and is not medical advice — especially at higher intakes.

Your health and profile data stays on your device. We only send your location coordinates to the weather service to fetch your local UV forecast. We don't sell your data. See our Privacy Policy for details.

Only with your permission. It can read things like your time in daylight, sleep schedule and age to personalize your plan, and write your estimated vitamin D and light-exposure sessions back to Health. You control this in your device settings.