Sunshine rhythms, a possible Zeitgeber for multiphasic biological rhythms during a year

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Abstract
Data on sunshine and barometric pressure were analyzed with a periodic regression technique, using the trigonometric fitting functions of the Fourier series. The data were obtained from U.S. Weather Stations in North Carolina, South Carolina and Virginia, and least‐squares fitted curves were obtained for each year's data from 1950 to 1968. A prominent similarity for most of the curves was the occurence of three or four peaks and troughs of approximately equal amplitude within a year. A peak in the fall of the year was usually present, and a length of day influence was not a typical feature of the curves. In general, then, sunshine and barometric pressure do not exhibit annual rhythms, and the possible role of these shorter term variations in climatological events as synchronizers of physiological processes is discussed. A review of studies where repeated sampling of endocrine or metabolic variables were made throughout a period of a year or longer indicates no annual rhythms but rather multiple peaks and troughs during the year like the sunshine and barometric pressure.