Circadian phase response curves for dark pulses in the hamster

Hamsters maintained under constant illumination were exposed to 2- or 6-h pulses of darkness at various phases of their circadian activity rhythms. When presented around the time of activity onset, the pulses resulted in phase advances, and when presented toward the end of daily activity, they resulted in phase delays. Since others have shown that light pulses presented at the same phases in constant darkness cause phase shifts in the opposite directions, these results indicate that phase response curves for light and dark pulses are mirror images. Dark pulses also caused phase-dependent changes, both transient and long-lasting, in the period of the free-running rhythms, and a few pulses were immediately followed by splitting of the activity rhythms into two components. Such effects may reflect a differential responsiveness of two coupled oscillators to dark pulses.