Sunlight falling on the skin can raise the level of sex hormones in the blood. This effect has been known for over fifty years, and has been the basis of, for example, measures to improve the laying rate of hens. But the same thing does happen in humans. When researchers gave doses of ultraviolet to subjects in Boston, USA, they found that a course of five doses, of increasing duration, each of them sufficient to produce slight reddening of the skin, could double the male hormone output.
This ties in with the studies which have shown that levels of testosterone, the major male hormone, rise by about twenty per cent through the summer, reaching a peak in September. In females, the effect was somewhat less but still measurable. The part of the body exposed to ultraviolet also made a difference. Some increase could be achieved whichever area of skin received the irradiation, but while exposing the back produced a doubling in hormones, exposing the skin of the genitals could cause the hormone level to triple.
3) Myerson, A., and Neustadt, R., 'Influence of Ultraviolet Irradiation upon Excretion of Sex Hormones In The Male', Endocrinology: 25; 7, 1939.
4) Aschoff, J., 'Annual Rhythms in Man', in Aschoff, J. (ed.), Handbook of Behavioural Neurobiology, Plenum Press, New York, 1981.