How Sunlight Makes Vitamin D — and How to Get It Right
Skin makes vitamin D when direct sunlight reaches it. It does not happen behind glass, and how long you need depends on skin tone, season and time of day.
Everyone knows sunlight makes vitamin D; the method is usually wrong. Sitting by a window, or walking in the shade fully covered, does not produce it.
How the body makes it
When certain wavelengths of sunlight reach bare skin, a compound in the skin begins converting to vitamin D. The liver and kidneys then turn it into the form the body uses.
Why glass blocks it
Ordinary glass filters out the wavelengths this depends on. Sitting behind a window, a car windscreen or a balcony pane gives you the warmth and none of the vitamin D.
How long, and when
- Start with gentle morning sun — it is kinder to the skin
- Some skin must be uncovered: arms, legs or back
- Darker skin needs longer to make the same amount
- Winter, and polluted city air, both lengthen the time needed
- Begin with short exposures and build up
No single figure fits everyone — which is why there is no "20 minutes a day" here. Your skin, your city and the season decide it together.
What about sunscreen?
Sunscreen blocks the same wavelengths, so it reduces production too. The practical approach is a short spell of morning sun without it, and sunscreen whenever you are outdoors for longer.
Frequently asked questions
What time of morning is best?+
Gentle morning sun is the easiest and safest place to start. Avoid the harsh midday sun.
Does sunlight work through clothing?+
No. The light has to reach skin, so something must be uncovered.
Is winter sun any use?+
Some, but less. The sun sits lower, so the same amount takes considerably longer.
Can you get too much vitamin D from the sun?+
No. The body regulates what it makes from sunlight. Excess comes from high-dose supplements.