This section is from the book "The Hygienic System: Fasting And Sun Bathing", by Herbert M. Shelton. Also available from Amazon: The Hygienic System Vol III Fasting and Sun Bathing.
A sun-bath taken at any time of the day will be beneficial and a busy person should take one at any time he or she can. But as the intensity of light and the length of time of exposure play important roles in sun-bathing, greater caution must be observed if a bath is had at mid-day in summer.
The early morning is the best time for sun-bathing, as at this time one may enjoy longer exposure without the depressing influence of intense heat. It is claimed in some quarters, though incorrectly, that the rays of the early morning sun are richest in ultra-violet rays. The late afternoon is also a good time for sun-bathing.
Rikli had his patients arise half an hour before sun-rise in the Summer and go up to the mountains and get their sun-baths during the coolest moments of the day. His baths were given on the mountain and it is here that the ultra-violet rays are most abundant.
Sir Henry Gauvin, English tuberculosis specialist, claims best results are obtained with sunbathing in tuberculosis, if there is also a current of air playing over the body. Cool breezes probably do more for the body than merely protect it from excess heat. If this is true, it only confirms the Hygienists' contention that air baths are of great value, even without the sun. Take your sun bath in the cool portion of the day, or else while the wind is blowing.
Dr. Lpeschkin, of the Desert Sanitarium and Institute of Research at Tucson, Arizona, calls attention to a fact with which everyone experienced with sunbathing must be familiar--namely, that sunlight is more likely to be enervating, even when it is not so hot, when it filters through a cloud, than when the unfiltered rays fall upon the body. He offers, as an explanation of this, the suggestion that whereas the visible rays of the sun often destroy the red cells, the unseen ultra-violet rays protect them from the other rays and strengthen them; so that when the ultra-violet rays are filtered out by a cloud and the other rays alone strike the body, the cells are unprotected.