OPSIN is your sunlight operating system. It models your whole light day — circadian rhythm, vitamin D, and burn safety — from one consistent engine, and tells you exactly when to step outside.
Most sun apps bolt together separate calculators, so they tell you to stop for safety while claiming you've barely made a dent in your vitamin D. OPSIN doesn't.
"Stop after 12 minutes — you've hit your limit." …and on the same screen: "You're at 5% of your vitamin D goal." Two numbers, two disconnected models, one confused user.
Your skin stops making vitamin D at about the same point sunburn risk begins — so "you've optimized your D" and "you've reached your safe limit" arrive together. When the sun can't get you there, we say so — and suggest a supplement top-up honestly.
Vitamin D and circadian health in one place — with the trackers the other apps make you pay attention to.
When to get bright light after waking to anchor your sleep, mood and energy.
The best UV hours today, with modeled IU and the safe number of minutes for your skin.
Your personal time-to-burn for the current UV, with a warning before you cross it.
A ranked daily plan — not just data — that says exactly when to step out and for how long.
Log supplements toward your daily target, keep a streak, and see weekly & monthly trends.
Estimate the light around you and check your indoor light is bright enough by day, dim by night.
Like a recovery score for your light — glanceable, honest, and built from real models.
Bright light early, dim light late — weighted for its effect on your body clock.
How close you are to your daily target from sun, supplements and diet.
Your overall light-quality-of-life — the one number that ties it all together.
Set up once, then open it each morning for the day's plan.
Skin type, age, where you are, your schedule, and any supplements you take.
Live, cloud-adjusted UV and sun position for your exact location.
The best windows to go out today, how long is safe, and how much vitamin D you'll make.
Log sun and supplements, build a streak, and watch your three scores trend over time.
OPSIN's engine uses the same photobiology researchers and doctors use — the UV Index, the erythemal/MED burn model, the vitamin D action spectrum and its synthesis plateau, and the Brown 2022 circadian-light consensus. We're transparent that every value is a modeled estimate, and we expose the assumptions.
Holick · Webb & Engelsen · Terushkin 2010 · Brown et al. 2022 (PLOS Biology) · CIE S 026 · Open-Meteo UV
Free to try in your browser. iOS app coming soon.
Launch the web app →OPSIN is a general wellness and lifestyle tracker. It is not a medical device and does not provide medical advice, diagnosis, or treatment. Vitamin D, UV and light values are modeled estimates, not measurements. Sun exposure carries real risks — always use your own judgment and consult a healthcare professional. See our Medical Disclaimer.