Circadian rhythms of animals are reset by exposure to light as well as dark, but while the parameters of photic entrainment are well-characterized, the phase-shifting actions of dark pulses are poorly understood. Here we determined the tonic and phasic effects of short (0.25h), moderate (3h), and long (6-9h) duration dark pulses on the wheelrunning rhythms of hamsters in constant light. Moderate and long duration dark pulses phase-dependently reset behavioral rhythms and the magnitude of these phase-shifts increased as a function of the duration of the dark pulse. In contrast, the 0.25h dark pulses failed to evoke consistent effects at any circadian phase tested. Interestingly, moderate and long dark pulses elevated locomotor activity on the day of treatment. This induced wheel-running was highly correlated with phase shift magnitude when the pulse was given during the subjective day. This, together with the finding that animals pulsed during the subjective day are behaviorally active throughout the pulse, suggests that both locomotor activity and behavioral activation play an important role in the phase-resetting actions of dark pulses. We also found that the robustness of the wheel-running rhythm was weakened, and the amount of wheel-running decreased on the days following exposure to dark pulses; these effects were dependent on pulse duration. In summary, similarly to light, the resetting actions of dark pulses are dependent on both circadian phase and stimulus duration. However, unlike lightmediated phase shifts, dark pulses seem to be complex stimuli, with photic and nonphotic resetting properties.