Vitamin D and sunlight: how much daylight do you need?

Technically, vitamin D is a precursor to a hormone rather than a classic vitamin. The special part: your body can make it with the help of sunlight. That makes it relevant for anyone who wants to stay fit and well over the long term.

Vitamin D helps your body absorb calcium, making it central to strong bones and teeth. It also supports muscle strength and the immune system. A lasting deficiency is often vague, with symptoms such as fatigue, muscle weakness or increased susceptibility to infection.

The catch: foods such as oily fish, eggs and fortified products provide only small amounts. Most of the rest depends on daylight. Next, we look at how your skin turns sunlight into vitamin D.

The decisive factor is the sun’s UVB radiation. When it reaches your skin, it converts a cholesterol precursor into vitamin D. Your liver and kidneys then turn it into the active form your body can use.

Whether this works depends mainly on the sun’s angle. When it is high in the sky, enough UVB reaches your skin. When it is low, the atmosphere filters out much of the radiation. That is why timing matters: during the warmer half of the year, production is highest between about 11 am and 3 pm.

It also matters what blocks the radiation. Glass, clothing and high-SPF sunscreen allow very little UVB through. Behind a window, or with your body fully covered, your body therefore makes almost no vitamin D, even when it feels warm and sunny.

The good news: it takes less than many people think. In midsummer, short sessions are often enough to fill your stores. As a rule of thumb, expose around a quarter of your skin, such as your face, hands and parts of your arms and legs, to sunlight several times a week.

Exactly how long depends on your skin type and the season. Fair skin makes vitamin D faster. Darker skin needs considerably longer. The balance matters: you want enough sunlight for vitamin D production, but no sunburn. Staying out longer than needed adds no benefit and increases the risk of skin cancer.

The table below offers a rough guide for unprotected skin around midday. It is only a rough measure, but it helps you get a feel for the timings.

In longevity circles, vitamin D is often marketed as a key to a longer life. The evidence is more restrained, and that is where a closer look pays off.

Its role in bones and muscles is well established: good vitamin D status lowers the risk of falls and fractures in older people. There are also plausible indications for the immune system. For active, independent ageing, that is no small thing.

The bigger promises, such as living longer and having fewer cancers or heart attacks, rest on thin evidence. The large 2019 VITAL trial found that additional vitamin D did not reduce cancer or major cardiovascular events. Some meta-analyses suggest long-term use may slightly lower overall mortality, but the results are inconsistent.

Vitamin D is an important building block. It is no miracle cure. Correcting a deficiency is beneficial. If you already have a good supply, extra tablets offer little additional value.

Winter is when many people start thinking about drops or capsules. They are most useful when you make little vitamin D from sunlight or belong to a risk group, such as older people, people with darker skin, those who rarely go outdoors or people who usually keep most of their body covered.

The German Nutrition Society (DGE) gives 20 micrograms (800 IU) per day as a guide when the body makes no vitamin D of its own. This matches the daily maximum for food supplements cited by Germany's Federal Institute for Risk Assessment (BfR). A low daily dose is preferable to occasional high-dose boluses. National recommendations can differ, so check the guidance where you live.

Before taking a high dose over a long period, check your actual level. A simple blood test at the doctor’s can measure your 25-OH-D level. That lets you supplement deliberately rather than guess, and avoids taking more than you need.

You do not need to track your daylight with precision. A few simple habits can get you through the year: sunlight on your skin in summer, with a little support in winter. Our Health check helps you assess your routines, while Outdoor Weather helps you find the best window for your time outside.