Synodic Monthly Modulation of the Diurnal Rhythm of Hamsters.

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Abstract
Four hamsters were monitored seriatim for hourly spontaneous activity over a 2-year period under artificially controlled conditions of light (6 a.m. to 6 p.m.) and dark (6 p.m to 6 a.m.). A large amplitude mean daily variation was found for each year with maximum activity at 7-8 p.m. and almost no activity by day. Total daily activity displayed each year a mean monthly variation of more than 15% with maxima each year occurring the 4th day after full moon and a day or 2 after new moon. Several arguments for concluding the monthly rhythm to be exogenous and responses to subtle geophysical variations are advanced.