Fact or Myth: Does Sunlight Through a Window Give Vitamin D?
Verdict: Myth. Ordinary window glass blocks almost all of the UVB light your skin needs to make vitamin D, so sitting in a sunny room feels warm and lovely but makes little or no vitamin D.
Before you pull up a chair by the window
- Verdict: Myth. Glass lets through light and heat, not the UVB that makes vitamin D.
- What is true: some UVA does pass through glass, which is why skin can still tan, age or darken behind a car window.
- What to do instead: short spells of direct sun on bare arms and face outdoors, a vitamin D blood test if you have symptoms or risk factors, and your doctor’s plan if you’re low.
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Why the window idea makes sense at first
Winter mornings in Lahore or Delhi. A patch of sun falls across the floor, and one by one the whole family drifts into it with cups of chai, a newspaper, a bowl of peanuts and the cat, until the brightest corner of the room is the most crowded place in the house, and because it so obviously feels like the sun is doing everyone good, it’s natural to assume the vitamin D part is happening too. Grandparents who can’t manage stairs to the roof are often seated by the window “for the sun,” with love and good intentions.
The warmth is real. So is the mood lift. The vitamin D, sadly, mostly isn’t.
How your skin makes vitamin D, explained simply
Sunlight is a mix of rays. Visible light lets you see, infrared is the warmth you feel on your face when you close your eyes in that winter patch of sun, and ultraviolet, which you can neither see nor feel at the time, comes in two main kinds that matter here: UVA and UVB.
Only UVB can switch on vitamin D making. When UVB lands on bare skin, it turns a form of cholesterol already sitting there into vitamin D, which your liver and then your kidneys convert, step by step, into the active form your bones, muscles and many other tissues actually use. No UVB, no vitamin D. Simple as that.
Normal window glass, in homes, offices and cars, blocks almost all UVB while letting a good share of UVA through, so behind glass you get the heat, the light and some of the skin-ageing and pigment-darkening effects of UVA, but not the one ray your skin actually needs to make vitamin D. Frustrating, but true.
Did you know?
Your shadow is a handy guide. When your shadow is shorter than you are, the sun is high enough for plenty of UVB to reach the ground. Early morning and late afternoon, when shadows stretch long, much less UVB gets through.
Myth: an hour in a sunny room is as good as sitting outside
It isn’t. However bright the room, the glass filters out the UVB before it ever reaches your skin, whereas opening the window wide and sitting in the direct beam is a completely different thing, because then the light isn’t passing through glass at all. A balcony, courtyard, roof or garden works; a sealed pane doesn’t.
A related belief: that people in sunny countries can’t be low in vitamin D. In fact low levels are common across South Asia. Dark skin needs longer in the sun to make the same amount, many people cover most of their skin, spend daylight hours indoors, and city smog cuts UVB too. Our guide to vitamin D in Pakistan explains why.
The part that is true
Window sun still has worth. Bright daylight in the morning helps set your body clock, lifts mood in the short, grey weeks of winter and can make it easier to fall asleep at night, so a seat by the window is still a kind thing to offer an elderly parent. The UVA that sneaks through can also darken melasma patches and age skin over years, which is why dermatologists suggest sunscreen even for people who sit by windows or drive a lot.
In many homes, newborns were once laid in the morning sun and elders sat out on the charpai in winter afternoons. Sitting outdoors in gentle sun is a habit worth keeping for adults. For babies, current advice has changed: keep babies under 6 months out of direct sun, and let the baby’s doctor decide about vitamin D drops.
What to do instead
- Go outside, or open the window fully, and let direct sun fall on bare forearms, hands and face, or legs where that’s comfortable.
- Aim for short spells near the middle of the day, when your shadow is shorter than you. Several short sessions a week help more than one long one.
- Stay well short of burning or going pink. Darker skin needs longer than fair skin, but burning is never the goal.
- Keep sunscreen for longer stays outside, and always for the face if you’re prone to melasma; short spells for vitamin D can be without it on the arms.
- Eat the foods that carry some vitamin D: oily fish, egg yolks, and fortified milk or cooking oil where available.
- If you feel constantly tired, have bone or muscle aches, or fall into a risk group, ask your doctor about a vitamin D blood test (called 25-hydroxy vitamin D).
- If the test is low, your doctor decides whether you need a supplement, which form and for how long. Don’t guess the amount yourself; too much vitamin D can build up and cause harm.
For people over 40, vitamin D works alongside calcium and strength training for bones; see bones after 40 on a desi diet. Other everyday claims are checked in Fact or Myth.
Who is most likely to be low
- People with darker skin, especially if they spend most daylight hours indoors.
- Women who cover most of their skin outdoors.
- Older adults, whose skin makes less vitamin D.
- Pregnant and breastfeeding women, and breastfed babies.
- People who are housebound, work night shifts, or have kidney, liver or gut conditions that affect absorption.
- People with obesity, as vitamin D gets stored in body fat.
Book a check for ongoing bone pain, muscle weakness, frequent cramps, or a fracture from a minor fall. In children, bowed legs, delayed walking or slow growth need a doctor’s review. Pregnant women and anyone on long-term medicines for epilepsy, steroids or kidney disease should ask their doctor about vitamin D rather than self-testing with sun or supplements. Your doctor decides any supplement; don’t start, stop or change one on your own.
Questions about window sunlight and vitamin D
Can you get vitamin D through a window?
Hardly any. Ordinary glass blocks nearly all UVB, the only part of sunlight that makes vitamin D in your skin. You need direct sun on bare skin.
Does sunlight through a car window give vitamin D?
No. Car glass blocks UVB like home windows do, though side windows can let some UVA through, which can still darken or age skin.
What is the best time for sun for vitamin D?
Around the middle of the day, when your shadow is shorter than your height. Morning and evening sun gives much less UVB.
How long should I sit in the sun for vitamin D?
Short spells, several times a week, without burning. Darker skin needs longer than fair skin. Because the right time varies with season, city and skin, there’s no single number that fits everyone.
Can I get vitamin D with an open window?
Yes, if direct sunlight falls on your bare skin through the open window. It’s the glass that blocks UVB, not the room.
Does sunscreen stop vitamin D?
In daily life, people who use sunscreen normally still make some vitamin D, because application is rarely perfect. Keep sunscreen for long sun exposure and the face.
Why is vitamin D deficiency common in sunny Pakistan and India?
Darker skin, covered clothing, indoor lifestyles, pollution and diets low in vitamin D foods all add up. Sunshine alone doesn’t guarantee good levels.
Should babies be put in the sun for vitamin D?
Babies under 6 months should be kept out of direct sun. Your baby’s doctor decides whether vitamin D drops are needed and how much.
How do I know if I’m low in vitamin D?
Only a blood test tells you for sure. Tiredness, aches and weakness can hint at it, but they have many other causes too.
Can you get too much vitamin D from the sun?
No. Your skin limits how much it makes. Too much vitamin D comes from supplements taken beyond what’s needed, which is why a doctor should set the amount.