AFTEREFFECTS OF SCHEDULED DAILY EXERCISE ON FREE-RUNNING CIRCADIAN PERIOD IN SYRIAN-HAMSTERS

This study examined whether a nonphotic factor, scheduled daily exercise, could cause aftereffects on the free-running circadian period of Syrian hamsters. Groups of hamsters were kept under a cycle of 14-h light: 10-h dark with access to their running wheel for only 3 h a day. Depending on the group, this 3-h period coincided with early day, midday, late day, early night, or late night. Controls did not have access to wheels. After 12 days, all hamsters were released into constant darkness (DD) and given free access to their wheel. Late-day runners showed a significantly shorter free-running period in DD compared to night runners and to controls, indicating that free-running periods can be shortened by nonphotic factors in this species. On the first day of DD, the activity onset of hamsters preceded (midday and late-day runners), coincided with (night runners and controls), or followed (early day runners) the previous time of D onset. Advanced activity onsets in late-day runners were consistent with both their short free-running periods and probable phase-advancing effects of late-day exercise; in contrast, delayed activity onsets in early day runners could only be explained by phase-delaying effects of the scheduled exercise.