Responses of the circadian locomotor rhythm to a single light pulse were examined in a diurnal rodent, Asian chipmunk, by exposing it to a 1-h light pulse of 2,000 lx under constant conditions. A light pulse given at the beginning and end of the subjective night produced a phase delay and advance shifts, respectively. When pulsed around the midpoint of the subjective night, the circadian rhythm was shifted as much as 12 h in most animals or became arrhythmic in some. In the latter case, an additional light pulse restored the circadian rhythm. Some animals were unresponsive to light. The phase response curve is categorized as type 0. A large phase-shift was sometimes followed by splitting of an activity band into two components. These results are best explained by an assumption that the chipmunk circadian system is composed of two mutually coupled major oscillators, each of which is constituted by multiple oscillators. Our results suggest that light affects the oscillatory coupling not only of the major oscillators but also of constitutional oscillators.

type 0 phase response curve; singularity; splitting; diurnal rodents; behavioral rhythm

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