Phase response curves to ambient temperature pulses in rats

The effect of pulses of warm ambient temperature on the phase of activity onset in Long-Evans hooded rats, Rattus norvegicus, free-running in constant light was examined. In two experiments, rats were exposed to pulses reaching a maximum of 34 degrees C or 32 degrees C. Phase response curves were obtained with advances occurring mainly in the subjective day, and delays mainly, but not entirely, in the subjective night. Significant negative correlations between rhythm period and phase-shifts were found. There were no consistent relationships between changes in activity levels due to the temperature pulses and phase-shifts. Cycles of higher and lower ambient temperature may entrain circadian activity rhythms in mammals by daily advance or delay phase-shifts.