ABSTRACT
For most species in which circadian rhythms have been studied, the pattern of ambient illumination exerts a powerful influence on the schedule of behavioral and physiological functions. This can be illustrated clearly in the adult white rat maintained on a regimen of alternating 12-h light and 12-h dark (LD 12:12) and housed continuously in a running wheel, with food and water supplied ad libitum and temperature held constant. With very few exceptions, more than 80% of the recorded activity occurs during the dark, and it is not unusual for the figure to reach 95–99%. Of the total ingestion, three-fourths or more typically takes place during these hours, and peak values of such functions as body temperature, adrenal cell mitosis, and susceptibility to certain stressors are registered. That the 346primary “zeitgeber” is illumination rather than some other, coincidental synchronizer is readily demonstrated by phase shifting the light-dark (LD) pattern forward or back by several hours while maintaining other conditions constant; the animals’ several rhythms begin promptly to shift to match the new environmental program, and even after an LD inversion the 180° readjustment is essentially complete in about a week.
