Phase shifts to refeeding in the Syrian hamster mediated by running activity.

Circadian rhythms in hamsters can be entrained by restricted daily feeding schedules. Phase control may be exerted by feeding per se, or by wheel running in anticipation of food access. Phase modulation by feeding was examined here by depriving hamsters of food for 9-24 h and refeeding at 1 of 7 different zeitgeber times on the first day of constant dim light. Significant group mean phase-advance shifts were observed only following 24 h and 17 h deprivations ending in the mid-subjective day, 7 h before the usual time of lights off (mean shifts 28 min and 66 min, respectively). The largest phase shifts were associated with wheel running during the first 6 h of refeeding. When running wheels were locked during this time in an additional group, no phase shifts were observed. A trend for small phase delays was evident for 14 h deprivations ending at the beginning of the subjective night, but no significant group mean or individual shifts were observed at other refeeding times. Refeeding after food deprivation, thus, appears to have minimal effects on circadian phase in hamsters; wheel running associated with refeeding may account for occasional shifts observed.