Effects of selected illumination levels on circadian periodicity in the rhesus monkey(Macaca Mulatta)∗∗

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Abstract
Aschoff (1960) recently stated a generalization called Aschoff's Circadian Rule which describes the effect of light intensity on circadian activity rhythms. The rule states that in light‐active animals the frequency of an activity rhythm, gross activity and the ratio of active to rest time should all increase with increasing light intensity. The research supporting Aschoff's Circadian Rule was based primarily on insects, birds, and rodents. The little research available on primates suggested that they may be exceptions to Aschoff's Circadian Rule. The present study was undertaken to determine the effects of light intensity on circadian activity rhythms of nonhuman primates. This was accomplished by subjecting four male Rhesus monkeys to a L/D 12/12 ratio, six different levels of constant illumination and constant darkness. The monkeys’ activity was monitored by ultrasonic motion detectors. The results clearly demonstrated that the Rhesus monkey is an exception to Aschoff's Circadian Rule. The period of the activity rhythm increased with increasing illumination and a behavioral threshold was evident at about 5.0 footcandles. Gross activity and the ratio of active to rest did not vary as a function of light intensity. It was concluded that the mediation of circadian rhythms in nonhuman primates probably differs from lower vertebrates.

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