In mid-February, a sunny window looks pretty appealing. You pull your chair into the light, feel the warmth on your face, and figure you’re at least getting some vitamin D. The same goes for a bright commute with the sun pouring through the windshield.
It’s a reasonable assumption, and it’s wrong. Sunlight through ordinary glass does almost nothing for vitamin D. Here’s why, and what to do about it for the rest of winter.
The Myth
“Sitting in a sunny spot indoors, or driving on a sunny day, helps your body make vitamin D.”
Why Glass Changes Everything
Sunlight contains several kinds of ultraviolet light, and they don’t do the same things.
- UVB (shorter wavelengths) is the part that matters here. When UVB reaches your skin, it converts a cholesterol-based compound into vitamin D3. Your liver and kidneys then turn that into the forms your body uses. UVB also causes most sunburns.
- UVA (longer wavelengths) goes deeper into the skin. It drives tanning, wrinkles, and other photoaging, and it contributes to skin cancer risk. It plays essentially no role in making vitamin D.
Standard window glass blocks nearly all UVB but lets a good share of UVA through. So in a sunny window you get the light, the warmth, and some UVA exposure, but almost none of the wavelength that makes vitamin D.
Cars work the same way. Windshields are laminated glass and block UVB along with most UVA. Side and rear windows also block UVB, but many let a meaningful amount of UVA through. That’s why long-haul drivers often show more sun damage on the side of the face nearest the window. A long, sunny drive gives your skin aging exposure without the vitamin D.
There are specialty glasses designed to let UV through, but you’re very unlikely to have them in your home, office, or car.
Winter Makes It Worse, Even Outdoors
Going outside doesn’t fully fix the problem this time of year either. How much UVB reaches the ground depends on how high the sun is in the sky. In winter at higher latitudes, the sun sits low and its light passes through more atmosphere, which filters out most of the UVB before it arrives.
The rough rule: above about 35-37° latitude, the skin makes little to no vitamin D from roughly November through February, even at midday with your face and hands exposed. In the US, that line runs roughly from Southern California across to North Carolina. Most of Europe and Canada sits well above it. Researchers call this period “vitamin D winter.” It runs longer the farther you are from the equator.
Add short days, heavy coats, and more time indoors, and it’s no surprise that blood vitamin D levels tend to bottom out in late winter and early spring. We explain who is most affected in our post on winter vitamin D needs.
What About Light Boxes and Tanning Beds?
Light therapy boxes, the 10,000-lux lamps used for winter blues, are built to filter out UV. That’s a safety feature. They make no vitamin D. They may still help with mood and sleep timing in some people, but that’s a separate effect.
Tanning beds mostly emit UVA. Some put out a little UVB, but they’re a poor and risky way to get vitamin D. Indoor tanning is linked to higher skin cancer risk, and dermatology groups consistently recommend against using it for vitamin D.
So Is a Sunny Window Useless?
Not completely. Bright daylight, especially in the morning, helps set your body’s internal clock and may lift energy and mood on gray days. Sitting by a window can be part of a good winter routine. It just shouldn’t count as your vitamin D plan.
How to Actually Cover Vitamin D in Winter
Food sources
Very few foods are naturally rich in vitamin D, but some help:
- Fatty fish such as salmon, trout, sardines, and mackerel are the best natural sources. A 3-ounce serving of salmon can provide several hundred IU.
- Fortified foods, including many milks, plant milks, some yogurts, orange juices, and cereals, typically add around 100 IU per serving. Check the label.
- Egg yolks have a small amount, roughly 40 IU each.
- UV-exposed mushrooms produce vitamin D2. Look for labels noting UV treatment.
Most people fall short of the recommended intake from food alone, especially in winter.
Supplements
- Recommended intake: 600 IU (15 mcg) a day for ages 1-70 and 800 IU (20 mcg) over 70. These are population targets. People with darker skin, larger bodies, limited sun exposure, or conditions that affect fat absorption may need more, which is worth discussing with a clinician.
- Common winter doses: 1,000-2,000 IU a day is a typical maintenance range for adults.
- Upper limit: 4,000 IU (100 mcg) a day for adults is the tolerable upper intake level. Higher doses should only be taken under medical supervision, usually after a blood test.
- Form and timing: Vitamin D3 (cholecalciferol) generally raises blood levels a bit more reliably than D2. Our vitamin D2 vs. D3 comparison covers the details. It’s fat-soluble, so take it with a meal that has some fat. Time of day matters less than taking it consistently.
Consider testing
A 25-hydroxyvitamin D [25(OH)D] blood test is the standard way to check your status. Expert groups don’t fully agree on what counts as “sufficient,” and different organizations use different cutoffs. A test still gives you and your clinician a baseline, especially if you’re at higher risk of deficiency. See our guide to blood tests before supplementing for what to ask for.
Safety Notes
Vitamin D is safe at recommended doses, but more is not better.
- Too much over time can raise blood calcium (hypercalcemia), causing nausea, weakness, confusion, frequent urination, and in serious cases kidney problems. This almost always comes from high-dose supplements, never from sunlight.
- Medication interactions: thiazide diuretics can increase the risk of high calcium when combined with vitamin D. Corticosteroids, some anti-seizure drugs, and the weight-loss drug orlistat can lower vitamin D levels or reduce absorption. Tell your clinician or pharmacist about any supplement you take.
- Check with a clinician first if you have kidney disease, a history of kidney stones, sarcoidosis or other granulomatous conditions, or a parathyroid disorder, or if you’re pregnant or nursing.
For the full picture on how vitamin D works, see our vitamin D explainer.
Bottom Line
A sunny window feels like sunshine, but ordinary glass blocks nearly all the UVB your skin needs to make vitamin D, while still letting UVA through. In winter at mid-to-high latitudes, even outdoor sun does little. Enjoy the daylight for your mood and sleep rhythm, but cover vitamin D with fatty fish, fortified foods, and, if needed, a modest supplement of about 1,000-2,000 IU a day with a meal, staying within the 4,000 IU upper limit unless your clinician advises otherwise.
This article is for educational purposes only and is not medical advice. Consult a healthcare provider before starting any supplement, especially if you’re pregnant, nursing, taking medication, or managing a health condition.