Feeding schedules and the circadian organization of behavior in the rat.

Feeding and drinking behavior of rats maintained in constant light were recorded before, during and after feeding schedules with periods lying within or outside the range of circadian entrainment. Regardless of period, all schedules immediately resulted in the partial or complete synchronization of drinking behavior, but failed to entrain the free-running circadian feeding and drinking rhythms. This indicated that drinking can be passively driven by periodic access to food. However, other results suggested that a separate circadian system was entrained by feeding schedules: First, the 24-h periodicity induced by 24-h feeding schedules often continued for several days after termination of the schedules. Second, the rats showed anticipatory activity under schedules with periods within, but not outside, the circadian range of entrainment. Third, lesions of the suprachiasmatic nuclei (SCN), which resulted in the immediate elimination of free-running rhythms, did not alter the rhythmic influences of the feeding schedules. These results provide evidence for the participation of two distinct circadian systems in the control of behavior in the rat. The two systems appear to have different entrainment characteristics and separate physiological substrates.