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.
Excess sunbathing proves to be very enervating and Tilden says he has seen patients who had so greatly enervated themselves by sunbathing, they were two years in recovering full nerve energy. I have seen much harm result from excessive stimulation of this kind. It is my advice to patients never to indulge more than an hour, and many cannot take this much sunshine. Referring to the claims of some places that they are lands of perpetual sunshine, Tilden says: "perpetual sunshine would add one more cause to enervation or to our already voluminous nerve-destroying cures and immunizations."
The stimulating effect of light is so well-known we need not dwell upon it here. The claim that sunbathing induces restful sleep and results in improved nerve tone is true only if the process is not overdone. Restlessness and decreased nerve tone result from overdoing it.