Dark adaptation in the circadian system of the mouse.

Entrainment of circadian rhythms by zeitgebers is generally believed to conform to the principles of the so-called nonparametric theory of entrainment. Although seldom recognized, this theory presupposes that the response of the circadian system to photic stimulation is dependent on previous photic stimulation. The process of dark adaptation in the circadian system has been investigated previously in only two studies in golden hamsters. In this study, dark adaptation in the circadian system of the mouse was investigated through the measurement of phase shifts of the locomotor activity rhythm of animals maintained in constant darkness for varying intervals of time before receiving single light pulses. It was found that 3 weeks of darkness are necessary for attainment of full responsiveness to light, whereas even a single 1-h pulse is sufficient to suppress the responsiveness. Additional results of this study indicate that phase shifts of the locomotor activity rhythm are consistently associated with changes in free-running period in both mice and golden hamsters. Phase advances elicited by single light pulses are associated with a shortening of circadian period, whereas phase delays are associated with a lengthening of period. Each 1 h of phase shift is associated with a period change of approximately 3 min.