Vitamin D
The Best Time of Day to Get Vitamin D from the Sun (Without Burning)
The midday paradox: peak UVB is the only way to make vitamin D, but also the fastest way to burn. How to get the dose without the damage.
Your skin makes vitamin D when UVB photons hit 7-dehydrocholesterol in your epidermis and convert it to previtamin D3. The catch: UVB only reaches the ground when the sun is high enough — roughly above 45° elevation — which means the same midday window that maximizes vitamin D synthesis also maximizes sunburn risk.
There is no morning or evening shortcut. UVA dominates outside the midday window, and UVA does not produce vitamin D. The honest answer is: you need midday sun, you need it on a lot of skin, and you need to limit it to well under your burn time.
Why morning and evening sun don't work
The atmosphere filters out UVB more than UVA at low solar angles. Below about 45° solar elevation, almost no UVB reaches ground level. In practical terms: outside the roughly 10am–4pm window, your skin can still tan and age, but it cannot meaningfully produce vitamin D. This is why people in northern latitudes are vitamin-D deficient all winter even on sunny days — the sun never gets high enough.
A simple shadow test works: if your shadow is shorter than your height, UVB is reaching you. If it's longer, it isn't.
How much sun you actually need
The classic figure cited by Holick et al. is that exposing 25% of body surface (arms, lower legs, face) to one-quarter of an MED produces about 1,000 IU of vitamin D — roughly the daily requirement. That works out to roughly:
- Type I — 5–10 minutes
- Type II — 10–15 minutes
- Type III — 15–20 minutes
- Type IV — 20–30 minutes
- Type V — 30–45 minutes
- Type VI — 45–75 minutes
Why this dosing keeps you well below burn
One MED is the threshold for visible redness 24 hours later. One-quarter of an MED is sub-erythemal — your skin gets the cholecalciferol-conversion benefit without DNA damage accumulating to the inflammation threshold. The DermUV Vitamin D Calculator uses this same logic: it gives you the minutes of midday sun for your skin type at the current UV, capped well below burn time.
Latitude, season, and the "vitamin D winter"
Above roughly 35° latitude (Atlanta, Albuquerque, Tokyo), the sun doesn't get high enough between November and February for meaningful vitamin D production. Above 50° (London, Berlin, Edmonton), the vitamin D winter stretches to nearly six months. Supplementation (typically 1,000–2,000 IU/day for adults, more for people with darker skin or limited sun) is the only practical option in those windows.
Sunscreen and vitamin D
Lab studies show SPF 30 reduces vitamin D synthesis by ~95%. Real-world studies consistently fail to find meaningful vitamin D deficiency in regular sunscreen users — likely because nobody applies enough sunscreen to get full SPF, and incidental face/hand exposure adds up. The honest take: sunscreen reduces but does not eliminate vitamin D synthesis. If you're a daily SPF user and your blood levels are low, supplement.
Diet and supplements
Fatty fish (salmon, mackerel, sardines) and fortified dairy provide modest amounts. For most adults outside the tropics, a daily supplement of 1,000–2,000 IU is the simplest and lowest-risk way to maintain serum 25(OH)D in the 30–50 ng/mL range that most endocrinology guidelines target.
Key takeaways
- UVB only reaches the ground when the sun is above ~45° — roughly 10am–4pm.
- 10–15 minutes of midday sun on arms and legs covers the daily requirement for fair skin; 30+ for darker skin.
- Above latitude 35°, winter sun cannot produce meaningful vitamin D.
- Supplementation is safer and more reliable than sun for hitting blood-level targets.