A novel, associative process modulating photic resetting of the circadian clock

Appropriate timing of physiological and behavioral processes requires that the circadian clock be reset daily by salient cues in the environment, particularly light. It is known that the ability of light to reset the clock depends both on its intensity and on the circadian time when it is applied (Daan and Pittendrigh, 1976; Moore-Ede et al., 1982). Here we show that the ability of a weak light stimulus to reset the clock is dramatically enhanced when it is presented daily at the same circadian time. Equivalent daily presentations of this light stimulus, but at different circadian times each day, do not lead to such enhancement. These findings suggest that the ability of light to reset the clock can be modified through a novel, and previously unrecognized, conditioning-like associative process in which circadian time serves as the conditioned stimulus and light as the unconditioned stimulus. The idea that circadian time can serve as a conditioned stimulus to modulate the effectiveness of light provides a new perspective on the lasting impact that light schedules have on the circadian clock and, thus, may have implications for existing models of photic entrainment (Pittendrigh and Daan, 1976; Moore-Ede et al., 1982).