Phase-shifting effects of single and multiple light pulses in the golden hamster

In a preliminary attempt to understand the mechanism of light adaptation in the circadian system, the phase-shifting effects of single and multiple light pulses on the rhythm of running-wheel activity were studied in golden hamsters. The results suggest that the reduction in sensitivity due to light adaptation (i.e., refractoriness to light stimulation) starts immediately (less than 1 min) after photic stimulation and that 40 min of stimulation are enough to saturate the system. Recovery of sensitivity can occur within 24 h or less if the photic stimulation is shorter than approximately 10 to 18 h but can take up to 3 weeks if the photic stimulation is longer. This suggests that, although photic stimulation for 40 min is sufficient to totally inhibit further phase-resetting, the process of light adaptation is fully completed only after 10 to 18 h of stimulation. An important implication of the results of this study is that the traditional concept of the phase response curve as a description of the resetting properties of the circadian clock is of limited use if light pulses are presented only at a single arbitrary interval after release of the animals into constant darkness.