Abstract
Running in a novel wheel can serve as a nonphotic zeitgeber to entrain or phase shift circadian rhythms in hamsters. In this study, hamsters were entrained to a light:dark schedule of 14:10 h but had no access to running wheels. At four different phase points of the light cycle, hamsters were transferred to constant darkness and provided with running wheels. All hamsters began running shortly after transfer and were allowed to continue running at their own volition. Approximately 20% of the hamsters transferred at zeitgeber time (ZT) 23 (ZT 12 = lights out) ran more than 4 h after transfer and showed phase advances of the circadian activity rhythm by as much as 15 h, while hamsters that ran less than 4 h on average did not phase shift. A similar result was observed for hamsters transferred at ZT 2. Hamsters transferred at ZT 5 and 8 also did not phase shift if they ran less than 4 h, although the relation between longer runs and phase shifts became less evident. A sustained run in excess of 4 h appeared to be associated with large phase advances. These results show that under certain conditions, a single sustained bout of wheel-running activity is capable of phase shifting the circadian pacemaker by more than 12 h.