Minutes outside with one tap, the vitamin D window from the real weather — and an honest answer when the sun isn't enough today.
The app calculates three things from your location and the day's weather — the sources are listed at the bottom of the page.
From the day's highest UV index, the app estimates between which times your skin can make vitamin D. Below UV index 3, practically nothing happens.7
The UV value comes from the weather service Open-Meteo for your location. The window is an estimate, not a measurement: clouds make it shorter.
The more skin is bare, the faster it goes. The app works with a quarter of the time until your skin reddens and follows the German Federal Office for Radiation Protection: two to three times a week is enough.5
Example: UV index 6, fair skin
At UV 6, fair skin reddens after about 33 minutes (WHO rule of thumb: 200 divided by the UV index).4 You'll find the calculator on the weather card on the home screen.
From UV index 3, the WHO recommends protection; from 8, you should avoid being outside at midday.4 The app tells you which sun protection factor fits today.
What the app recommends
Gold means: avoid the midday sun. Always avoid sunburn — vitamin D included.
+10 or +20 minutes or your own time — one tap is enough. “Best window” shows you when the sun does the most today. If you wear an Apple Watch, your time in daylight from Apple Health is added automatically.
Apple Health provides “Time in Daylight” from iOS 17. Your own entries stay untouched.
Light through your eyes sets your inner clock to the day; light late in the evening pushes it back.8 The app knows when you get up and reminds you of the first hour after.
The 10 minutes are a guide value of the app, not a number from a study. Light counts on cloudy days too — just don't look straight into the sun.
How many minutes were you outside, and on which days did you reach your goal? After four weeks you see your pattern — for example, on which weekdays you get out least.
The daily goal of 20 minutes is a guide value of the app, not a number from a study. You can change it yourself.
From October to March, UV-B radiation in Central Europe is too weak, and skin makes hardly any vitamin D.6 The app tells you so instead of calculating a window that doesn't exist.
In the Supplements area, the app suggests vitamin D3 in these months — based on the season. Only a blood test shows whether you're really low.
The app already calculates these connections today. A connection is not a cause — but it shows you where to look.
If you were outside in the first two hours after getting up, it shows as a “morning light” point on the cortisol card in the Stress area.
From ten shared days, your history places sleep and time outside in a scatter chart — one point per day. A connection, not a cause.
Your history also compares training and sun from ten shared days, so you can see whether your active days are also your bright ones.
Small amounts of UV radiation are needed for the body to make vitamin D. The WHO notes that people who get very little sun can consider a supplement.12
In a small study with 14 men, one hour of bright light (800 lux) early after waking raised the cortisol response after waking more strongly. In the evening the light had no such effect. Cortisol is the hormone that gets you going in the morning.3
The WHO advises sun protection once the UV index reaches 3 or above, and limiting time in the midday sun.1
The WHO advises against artificial tanning devices. They raise the risk of skin cancer and should not be used to get enough vitamin D.1
A new article every Friday.
Biohacking · Light & ChronobiologyMorning light: How early light exposure sets your biological clockAnyone who checks their screen first thing in the morning is missing out on a free, powerful biohacking tool: direct daylight. How early light exposure anchors your circadian rhythm — and why it does more than any sleep supplement.Read more
Nutrition & Supplements · MicronutrientsVitamin D in winter: do you really need supplements?From October to March the sun in our latitudes is too low for the skin to produce vitamin D. What this means, who is affected and when supplements make sense.Read more
Biohacking · Light & SleepBlue Light & Melatonin: How screens affect your sleep hormoneAlmost everyone checks their smartphone or laptop in the evening — but what does that light do to your sleep hormone? How blue light suppresses melatonin production and what you can do about it.Read moreEvery statement comes with its source; the figures are taken from the linked studies and opinions (checked on 1 November 2026). This is context from the research, not a diagnosis and no substitute for medical advice.
Get YUTHI from the App Store — iPhone first, from November 2026.
