In order to test the possibility of a change in stress susceptibility induced by a forced change in activity rhythm, during control weeks food availability was restricted to the dark period only (fa-D) and to the light period only during the test week (fa-L). Stress susceptibility was measured on the basis of the decrease in heart rate (bradycardia) and an increase in freezing behavior in response to a sudden reduction in background noise. A significant increase in bradycardia both in magnitude and duration was observed during fa-L conditions as compared to the control fa-D condition. Also the duration of freezing behavior was increased during the fa-L condition. These results support the hypothesis that forced changes in circadian activities affect stress susceptibility. The significance of this finding for the incidence of stress pathologies is discussed.