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.
Sunbathing is entirely different from the popular practice of enjoying the fresh air. The bath is taken with all of the clothing removed. Care must be taken not to burn the body. Too little, rather than too much, should be the rule. Blondes and red-haired people must be more careful than brunettes and members of the dark races.
Begin the sun-bath by exposing the entire body six to ten minutes a day and gradually increase the length of time of exposure until half an hour to an hour or more, even to three and four hours are consumed. Make haste slowly. Expose the front of the body three to five minutes and then, expose the back three to five minutes. While I often find that even this rate of increase in the time of exposure is too fast, and am forced to slow it down considerably, I do not think that this rule needs to be followed closely by the well and active person. But where one is lying in the solarium, he may more easily over-do sunbathing than when romping on the beach. I believe that more benefit is derived from exposure of the back to the sun than from exposure of the abdomen. I cannot prove this at present. It is only a private theory.