Why is sunshine healthful?

 Everybody has a kind of instinctive feeling that being in the sunshine is good for us. But did you know that there are specific scientific reasons why this is so? Let's consider some of the things that happen when we expose our bodies to the rays of the sun.

Sunlight actually destroys certain fungi and bacteria that may have settled on the skin. It acts as a pretty strong medicine in this way!
Another way in which it acts as a medicine is by causing the white blood cells, or phagocytes, to become more active. These are the cells which attack disease germs in our body and help keep us healthy.
When sunlight strikes the skin, it causes it to send substances into the blood which gives the muscles new tone. The muscles become more tense and thus can work better. In fact, our nervous system gets a kind of “charge" from sunlight so that we feel stimulated and want to move about. This is one of the reasons why we're always so active on our vacations. We want to play ball and do exercises and go swimming. We want to move about more because the sun has helped wake up our nervous system.
The sun also has the very specific effect in that it creates a vitamin in our body. The ultraviolet light transforms a substance called ergosterol in the skin into vitamin D, which is called "the sunshine vitamin."
These are some of the reasons why we can say that exposing ourselves to the sun's rays is like swallowing a teaspoonful of medicine every five minutes. But it's important to remember that this medicine is by no means harmless! We have to be just as careful about sun bathing as we would have to be about taking medicine.
Sun bathing raises the blood pressure, so people who have heart trouble or certain lung diseases can be injured quite seriously by exposure to sunlight. In fact, everyone should expose himself to sunlight only in stages. In taking sun baths, for instance, it's a good idea to start by exposing only one fifth of the body for five minutes. The next day, expose another fifth of the body for an extra five minutes, and so on. Your
doctor can give you the best advice on how to get all the benefits of sunlight without taking unnecessary risks.

Source: A book titled More Tell me Why