We can't believe we actually have to say this, but being regularly exposed to sunlight is an important part of being healthy.
We need get sun on our bare skin to allow our bodies to make Vitamin D, an essential nutrient that helps build strong bones, well-functioning muscles, and a strong immune system. It’s hard to get enough from food, and we can usually only get enough if we are eating some that's fortified like milk or cereal. Plus, it's tough to tell how much sunlight is doing you good and not harming your skin, since guidelines about how much we really need to be our healthiest have varied over the years.
A recent study from the Solar Radiation Research Group at the Polytechnic University of Valencia in Spain examined the question and came up with some answers. They studied people with skin type III, which is most common among Spaniards—also prevalent across North America—and is classified as skin that will “tan easily, but still sunburn.” The researchers found that in spring and summer, people only need 10 to 20 minutes of sunshine to get the recommended dose of vitamin D. Too much longer will cause erythema—aka the start of a sunburn.
For the fall, about 30 minutes is sufficient. In the winter months, however, when you're most likely to be hibernating under your covers for hours on end, you need to be in the sun for 150 minutes. “The problem can appear in winter due to low levels of UV radiation and because people cover most of their bodies," said lead study author María Antonia Serrano, Ph.D.
She also cautions that “the figures would change for those who are lighter or darker in complexion. It is also essential to bear in mind that we have considered the usual percentage of the body exposed for the season. If more skin is exposed, exposure time can be reduced.”