Circadian Clock in Photoperiodic Time Measurement: A Test of the Bünning Hypothesis

A technique has been developed for effectively separating the direct inductive effect of a light signal from its effect on the phase of the rhythm of sensitivity to photoperiodic induction. With this technique it has been shown that a 75-minute pulse of light per day, when appropriately positioned with respect to the circadian activity cycle of the sparrow Passer domesticus, is sufficient to produce a response normally produced only by long days. The results cannot be interpreted in terms of a requirement of an absolute amount of either darkness or light and offer strong confirmation of Bünning's hypothesis concerning the mechanism of photoperiodic time measurement.