Solar and Lunar Rhythmicity in the Rat in ‘Constant Conditions’ and the Mechanism of Physiological Time Measurement

¹ From the Department of Biological Sciences, Northwestern University, Evanston, Illinois

The spontaneous activity of one male rat under constant conditions was recorded for 120 consecutive days. During the first 70 days in constant illumination of 1 fc the 12-hour daily period of activity occurred regularly about 1 1/4 hours later each day, with the period scanning the solar day about four times during the 70-day period. During the succeeding 25-day period in darkness the daily cycles averaged exactly 24 hours with the time of day of activity, that of the last day in constant light. This was followed by 8 days in constant light followed by 18 days in constant darkness with completely comparable results. The daily running cycle randomized relative to the hours of the solar day, exhibited a daily cycle of amount of activity at each hour of the solar day, and randomizing both the daily activity period and the solar-day basic cycle revealed a cycle of lunar-day length, with minimum of lunar zenith and a maximum at nadir. There were also strong suggestions in the mean daily activities of 27-day and synodic monthly cycles.

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