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 advice quotes one number. The honest answer moves by a factor of thirty-five depending on where you are, when it is, your skin and how much of it is uncovered.
The variables that actually decide it: UV index (a UV Index of about 3 is the working threshold — below that there is little useful UVB whatever the temperature), latitude and season (above roughly 35–40° there are winter months when the midday sun cannot make vitamin D at all), skin tone (deeply pigmented skin may need up to ten times longer than very fair skin), how much skin is uncovered, and age (older skin holds less of the 7-DHC precursor).
For scale: modelling at 52°N puts the clear-sky time to make 1,000 IU at 1.1 minutes on 21 June and 39 minutes on 21 December — and effectively impossible under December cloud. Synthesis also self-limits well before your skin reddens, so longer is not better.
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.
Need vs intake vs safe-sun vs gap — and an estimated blood level that calibrates to you when you log a lab test.
Ask "should I go out now?" and get answers grounded in your own data — with strict safety guardrails, never medical advice.
Log supplements and lab results toward your target, keep a streak, and see weekly trends.
On iOS, estimate the light around you and check it's bright 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.
And we show our work. Our science library is a 19,000-word cited review of what light actually does to the body — vitamin D as a hormone, the contested blood-level targets, the red and near-infrared half of sunlight, the body clock — with an honest evidence grade on every claim, the null results included.
Holick · Webb & Engelsen · Terushkin 2010 · Brown et al. 2022 (PLOS Biology) · CIE S 026 · Open-Meteo UV
Free to try in your browser. The iOS app — with the light meter, HealthKit sleep proof, and notifications — is 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.