Control by light of the temperature rhythm in food-restricted hamsters.

To establish the relative importance of light and food in the control of core temperature (Tc) rhythm in food-restricted hamsters, mature female hamsters maintained in 14L:10D lighting were fed restricted amounts of food at the onset of light (n = 6) or at the onset of dark (n = 6) and were compared to ad lib-fed animals. After 21-25 days of this entrainment, light stimulus was shifted by 12 h, and animals were kept in shifted lighting for another 13 days. Food restriction led to a 0.6 degree decrease in the mean Tc, which was expressed entirely during the day in night-fed hamsters and was evenly divided between day and night in day-fed animals. Thus, Tc and general activity rhythms maintained the entrainment to light under both dietary conditions, with peak values for all occurring during the early night. During 13 days following the 12-h shift in lighting, Tc and activity rhythms shifted in all animals, regardless of nutritional status, from entrainment to preceding lighting, through double rhythm frequency, indicating entrainment to preceding as well as current lighting, to entrainment to current lighting. Thus, in food-restricted hamsters, light stimulus rather than predictable timing of food prevails as the entrainer of Tc and activity rhythms.