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.
The devitalizing influence of the hot sun is well known. People who lounge on the sand at the beaches at winter or summer resorts become lazy and indifferent, when they could, by moderate indulgence in sun bathing, and by cultivating less depressing activities, attain to greater vigor.
We must distinguish between the "light" of the sun and the heat of the sun. It is not the sun's heat from which all these benefits flow. Cities like Chicago and Pittsburgh receive plenty of the sun's heat, but less of its light, or less of its non-luminous rays, with the result that the blood of their inhabitants is on an average, about twenty per cent deficient in hemoglobin.
Animals seek the sunlight but avoid its heat. This is to say, they prefer to be in the sun during the cool portions of the day and seek the shade when it grows hot. The extreme heat is depressing and enervating. The guiding hand of animal instinct in avoiding the heat of the sun may be seen in the city's zoological gardens, the country pastures, or in the untamed places of the earth. The Indian in Mexico, Peru, South America, the Negro in Africa, all obey this instinct. The fox, the chamois in Switzerland, the cows in the pasture, the hens in the barn lot, the birds in the tree tops all love to bask in the sunlight of morning, but retreat to the shade as the heat of mid-day approaches.
In taking a sun-bath, heat is rather to be avoided than sought after. A temperature of 64 degrees F., being most suitable. Above 85 degrees F., prolonged exposure to the heat becomes enervating. Below 60 degrees F., the bath is still very beneficial.
In the tropics the leaves of palms and trees are either thick and heavy or have their edges turned toward the sun. At mid-day in Summer, when the sun it hottest, the leaves of plants curl up. Like birds, insects and beasts, the plant escapes the excessive heat of the sun as much as it can. Like the lower animals, led by their unerring instincts, we should obtain our sunbaths during the cool portions of the day.