Vitamin D and Time of Day
The sun's height above the horizon (solar zenith angle) controls how much UVB reaches the ground. At sunrise and sunset the sun is low, UVB traverses more atmosphere, and gets absorbed almost entirely. At solar noon, UVB peaks. The window where meaningful vitamin D synthesis occurs is much narrower than the total daylight window.
The 35° rule
The sun needs to be at least ~35° above the horizon for usable UVB to reach the ground. Below that angle, the atmospheric path length filters out virtually all UVB while transmitting UVA (which tans but doesn't make vitamin D). This is why sunrise and sunset feel gentle — they are. And why morning coffee in the sun makes no vitamin D.
Solar noon hits differently by latitude and season
- Equator (0°) at any equinox — sun straight overhead at noon (90°).
- 40° N in summer solstice — 73° at noon; strong UVB from about 10 AM to 3 PM.
- 40° N in winter solstice — 27° at noon; sub-threshold; no meaningful UVB all day.
- 60° N (Oslo) in summer — 53° at noon; usable UVB in a narrower window.
- 60° N in winter — 7° at noon; effectively no UVB.
The vitamin D window
At temperate latitudes in summer, the vitamin-D-synthesising window is roughly 10 AM to 3 PM (some conservative sources say 11 AM–1 PM). At the winter solstice above 40°, the window is closed entirely. In spring and autumn, it opens for 1–3 hours around noon in temperate zones. Adjust your outdoor time accordingly if you're relying on sun for any of your vitamin D.
Morning walks and evening jogs
Great for cardiovascular fitness, mood, and circadian entrainment (early morning light regulates the biological clock). Not effective for vitamin D synthesis. If you exercise outdoors early or late, you still need to supplement or get midday sun for vitamin D.
Daylight saving time confusion
Solar noon (when the sun is highest) is when it's highest — not necessarily 12:00 PM on the clock. During Daylight Saving Time, solar noon in most US locations falls around 1:00 PM. When people say "midday," they usually mean solar noon ± 1–2 hours. Check a solar-position app if you want to be precise.
Shadow rule of thumb
If your shadow is shorter than your height, you're in usable UVB territory. If your shadow is longer than your height, the sun angle is too low for meaningful vitamin D synthesis. A crude but functional field guide.
Practical timing
- Aim for outdoor time between 11 AM and 2 PM in summer for vitamin D.
- Fair skin: 5–15 minutes of face and arms is often enough. Stop before pinkening.
- Darker skin: 20–45 minutes may be needed for equivalent synthesis.
- Winter above 40°: don't rely on any time-of-day; supplement.
- See our best time of day page for the sun-vs-supplement timing question.