That is why children should be kept in the soft sunlight in the morning.

I've been wondering about the best time to get sun and how long we should expose our skin to the sun.

Apparently our skin produces something called 7-dehydrocholesterol. UV B rays from the sun turn this into vitamin D3. (NOTE: UV B rays also cause skin cancer).

The atmosphere filters out the UV B spectrum. It does not pass through windows. This means that UV B is strongest at noon.

In the winter it is best to catch some sun at noon when the UV B is most intense.

The big question that I have not been able to answer is how long? I've been thinking one needs about 5 to 20 minutes. But the duration could be shorter or longer.