Here’s something that trips up a lot of people: you’re sitting by a sunny window, light is pouring in, it feels warm, and you assume your body is doing its vitamin D thing. It’s not.
Glass blocks the specific type of ultraviolet light — UVB rays — that your skin needs to actually produce vitamin D. Regular window glass filters those rays out almost entirely. The warmth you feel? That’s infrared radiation passing through just fine. But warmth isn’t what makes vitamin D. UVB is. And UVB doesn’t make it through the glass.
So that cozy window seat, your car ride home, the sunlight streaming into your classroom — none of it is doing what you might think it’s doing for your vitamin D levels.
To actually make vitamin D, your skin needs direct exposure to sunlight outdoors. The amount of time that takes varies a lot depending on your skin tone, where you live, the time of year, and the time of day. Darker skin tones need more time in the sun to produce the same amount of vitamin D as lighter skin tones. In winter months, especially if you live far from the equator, the sun’s angle means UVB rays barely reach you at all — even if you go outside.
And here’s the thing about vitamin D: it matters more than people realize. According to the Mayo Clinic, your body needs vitamin D to absorb calcium properly — which means it’s directly tied to building and maintaining strong bones. It’s not just some bonus nutrient.
Vitamin D deficiency is more common than most people expect, especially in teens and young adults who spend a lot of time indoors. If you’re not getting regular outdoor sun exposure and you’re not eating foods high in vitamin D (like fatty fish or fortified milk), your levels might be lower than they should be.
The fix isn’t complicated. A little time outside — actually outside, not just near a window — goes a long way. So does eating vitamin D-rich foods or talking to a doctor about whether a supplement makes sense for you.