Vitamin D isn't really a vitamin: it's a pro-hormone that your skin synthesizes from UVB rays. It is essential for calcium absorption, immune function and mood regulation. Yet in Northern Europe and the US, nearly one adult in two is deficient.
How long in the sun?
The required time depends on three factors: your skin type (Fitzpatrick scale), the current UV index, and the area of skin exposed. Reference table to reach roughly 1000 IU with arms and legs uncovered, at UV index 6:
| Skin Type | Description | Exposure time |
|---|---|---|
| Type I | Very fair, always burns | 10 min |
| Type II | Fair, often burns | 15 min |
| Type III | Medium, tans gradually | 20 min |
| Type IV | Olive, tans easily | 30 min |
| Type V | Brown | 40 min |
| Type VI | Black | 60+ min |
How much is too much?
Beyond the optimal synthesis window, additional exposure produces no extra vitamin D but multiplies the risk of sunburn and skin cancer. That's exactly when Sun Day sends an alert: "you have your dose, protect yourself now."
Winter: do you need supplements?
From November to February above the 45th parallel (Bordeaux, Lyon, Boston, Seattle), UVB rays are virtually absent. Skin synthesis is impossible and oral supplementation is generally recommended. Sun Day tells you in real time whether the UV index allows synthesis or not.
How Sun Day calculates your dose
The app combines your skin phototype, the percentage of skin exposed (you can adjust it), the UV index measured at your location, and your age to estimate your IU intake minute by minute. Calculation is fully on-device — your data never leaves your phone.
Track your vitamin D every day
Sun Day is free, ad-free and respects your privacy.