Overview
- Guidance across outlets points to late morning to early afternoon—roughly 11 a.m. to 2–3 p.m.—as the most effective window for UVB-driven vitamin D synthesis, with morning sun yielding far less.
- UCLA-cited estimates suggest about 8–10 minutes at noon in spring or summer with roughly 25% of skin exposed, compared with nearly two hours at noon in winter when only about 10% is uncovered.
- Winter’s low sun angle and factors such as latitude, darker skin, clothing, and sunscreen often make sunlight alone insufficient, prompting tactics like exposing arms and legs briefly in sheltered sunny spots.
- Dietary sources including fatty fish, eggs, mushrooms, fortified dairy or plant milks, and cod liver oil are recommended, with supplements advised under medical guidance when needed.
- Safety guidance underscores a 4,000 IU per day upper limit from Harvard Health and notes that some adults may require higher medically supervised doses, while a simple cue—shorter personal shadow—signals stronger UVB.