You’re sitting under a tree on a bright day, warm light all around but no beam of sun on your skin, and you wonder: does this “indirect” sunlight still help your body make vitamin D?
What your skin actually needs
Vitamin D production begins when ultraviolet-B (UVB) wavelengths in sunlight hit a cholesterol-derived molecule (7-dehydrocholesterol) in the outer layers of skin. That kickstarts a chain of conversions that ultimately yields the circulating form measured in blood, 25-hydroxyvitamin D.
Two practical points follow from the chemistry. First, UVB, not visible light, drives the process, which is why brightness alone doesn’t guarantee vitamin D production. Second, your skin needs a dose, not a perfect beam: lower-intensity UVB for longer can add up to the same “UVB dose” as a short, stronger burst. Production also self-limits; beyond a point, additional UVB degrades precursors and the vitamin D yield plateaus before you burn.
Clouds, shade, and the UVB that slips through
On cloudy days, enough UVB often reaches the ground to keep vitamin D synthesis going, just more slowly. Geophysics research has estimated that overcast skies typically cut surface UV to roughly 30–70% of clear-sky levels, depending on cloud thickness, height, and coverage. Broken cloud can even spike readings briefly as light scatters off cloud edges.
Shade is different. When you step under a tree or umbrella you block almost all direct UVB, but you’re still bathed in diffuse UV scattered by air molecules and reflected from the ground, walls, and nearby surfaces. In practice, shade often reduces UVB exposure by half or more, but not to zero. That’s why people can get mild sunburn in bright shade at the beach, and why vitamin D synthesis can still occur—just more slowly—when you’re not in a direct sunbeam.
Indoors is another story
Regular window glass filters out nearly all UVB. UVA, which tans and ages skin, passes through; UVB, which is needed for vitamin D, does not. Sitting by a sunny window may feel wonderful, but it won’t move your vitamin D levels. Artificial indoor lighting generally lacks UVB as well unless it’s a medical UVB device.
If you’re under a shaded awning outdoors, you’re in a sea of diffuse UV and can still make vitamin D; if you’re behind glass indoors, you usually can’t.
How long does “indirect” take?
Time-to-D depends on the UV Index, season, latitude, elevation, skin pigmentation, age, and how much skin you uncover. Midday has the most UVB because the sun’s path is shortest through the atmosphere. In summer at mid-latitudes, experts generally place sufficient outdoor exposure in the range of minutes to tens of minutes to arms and legs, a few times per week; under clouds or shade, similar benefits can take longer.
Skin tone matters. Higher melanin levels absorb UV, protecting against DNA damage but also slowing vitamin D production; darker skin typically requires longer exposure than lighter skin to achieve the same blood level. Season matters too: above roughly 50 degrees latitude, little to no cutaneous vitamin D is produced in winter because the sun sits too low in the sky and UVB is filtered out.
Sunscreen, myths, and real-world vitamin D
Laboratory tests show that a thick coat of sunscreen can block UVB on the skin surface. But how does that translate to daily life? A 2019 systematic review in the British Journal of Dermatology examined experimental studies, randomized field trials, and dozens of real-world observational studies. Its bottom line: routine use of typical sunscreens didn’t meaningfully lower vitamin D status in outdoor settings.
“There is little evidence that sunscreen decreases 25(OH)D concentration when used in real-life settings.”
Subsequent reviews have echoed the nuance: sunscreen reduces UV exposure and the risk of burning, but people still get enough incidental UVB for vitamin D when they spend time outdoors, even with daily application. One recent review concluded that sunscreen “only minimally lowers vitamin D production” while still cutting skin cancer risk.
“Sun protection strategies should still be implemented to reduce skin cancer, and sunscreen only minimally lowers vitamin D production.”
Two practical caveats remain. Very high SPF, applied generously and often, might lower vitamin D more than typical use—there’s less evidence here. And if your UV exposure is already limited by season, latitude, clothing, or mostly-indoor life, sunscreen is not the main reason your vitamin D runs low.
Diet, supplements, and a sensible mix
Few foods naturally contain vitamin D in meaningful amounts. Fatty fish, cod liver oil, egg yolks, and fortified milk or cereals help, but diet alone often falls short. Observational data in the United States show that many people who meet vitamin D blood targets do so with a combination of sun and fortified foods or supplements rather than diet alone.
When UVB is scarce—deep winter at high latitudes, or for people who must avoid sun—supplementation is a straightforward backstop. Meanwhile, brief, non-burning outdoor time remains a cost-free way to complement diet when conditions allow. The key is to seek light responsibly and avoid chasing a tan.
Putting it all together outside
Indirect sunlight can still power vitamin D synthesis, thanks to diffuse UVB that filters through clouds and scatters in shade. But expect it to take longer than a clear, direct midday exposure. Use the UV Index as your guide: higher numbers mean a shorter time is needed; lower numbers mean more time.
- Clouds usually cut UV to 30–70% of clear-sky levels; shade often halves it again. Plan more time, not more risk.
- Outdoors under shade beats indoors by a window for vitamin D. Glass blocks UVB.
- Midday is most efficient; aim for brief, non-burning sessions with uncovered arms and legs.
- Adjust for skin tone, season, and latitude; consider supplements when UVB is scarce.
- Wear sunscreen to prevent burns; real-world evidence suggests it won’t erase your vitamin D.