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.
As might have been expected, any influence which produces such marked effects upon nutrition and occasions such profound changes in the superficial as well as the deeper tissues of the body, as does sunlight, exerts a wholesome influence upon the mind. It is a matter of common observation that on dark, cloudy days, people are more subject to worry, ill-temper, moroseness, the "blues," etc., and that as soon as the skies become clear again and the sunshine returns, happiness and good naturedness return. But the sun's influence strikes deeper than this.
Dr. James C. Jackson noted nearly eighty years ago that "the more a man lives in sunlight, other things being equal, the more vigorous will his brain be; the more vigorous this, the more energetic and competent to their office will his mental faculties be."
A class of boys from the slums of London were taken to the garden of a private home on Clapham Common, where they studied and played all day attired only in very short "shorts" and no shirt. At the end of six weeks, in the feeble light afforded them by smoky, foggy London, they showed an increase in mental capacity and alertness.
Comparisons were made of physically defective children of London with physically defective children who had received light treatment at the Lord Mayor Treloar Cripple's Hospital at Alton. Both groups were mentally retarded because of their afflictions, and both were about the same age, eleven years. The London children had had more schooling. The mental retardation of the London children averaged 1.95 years while that of the Alton children averaged 1.14 years.