A negative correlation between sunshine and conceptions in the USA, 1967–1972

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Abstract
Six years of data on hours of sunshine and number of births in 47 states of the USA were inspected beginning in January, 1967, for sunshine, and in October, 1967, for births. For each of the states, jackknife estimates of the cross‐correlation were obtained. Statistically significant and negative correlations were found in most states with a general trend of a weaker relationship in more northern latitudes. The intensity of the sunlight increases with decreasing latitude, matching the increase in the range of births that occurs at lower latitudes. Other seasonal rhythms in humans are briefly reviewed, and the ubiquity of circadian rhythms and photoperiodism in both plants and animals is noted. In mammals, seasonal events are controlled by melatonin, and the possible relevance of this control system to humans is discussed.