For centuries, men have recognized the therapeutic action of sunlight and have taken for granted that sunbathing increases man's energy and sense of well-being. Heliotherapy, however, was first advocated toward the end of the 18th century by Faure for chronic ulcers of the skin and by Chauvin for relief from apathy and anemia.
In 1877 Downs and Blunt discovered the bactericidal action of sunlight, and at the end of the century Finsen established that this action, as well as the production of sunburn, was caused by the invisible ultraviolet rays of the sun. With artificial light that imitated the solar spectrum and with sunlight, he and Bernhard pioneered in the successful treatment of diseases of the skin and other organs. In the 20th century the biochemical and physical properties of sunlight have been determined.
Apparently the conservative or rational use of sunlight or its artificial substitutes is healthful to most
Kesten BM. THE EFFECTS OF SUNLIGHT ON THE SKIN. JAMA. 1956;161(16):1565-1567. doi:10.1001/jama.1956.62970160003011