Two possible determinants of the timing of daily episodes of behavior in rats

The present experiment examined endogenous ultradian rhythms and regulatory metabolic processes as two potential determinants of daily out-of-nest episodes (ONEs) and feeding episodes (FEs) in rats living in a 24-h environment. Two types of photoperiod were used: a Standard 12:12 h L/D schedule and a Skeleton 12:12 h schedule. During each type of photoperiod rats were exposed to 4 feeding conditions of 7 to 12 days each: (a) a baseline of ad lib food and water; (b) restricted to the diurnal (inactive) portion of the cycle; (c) both food and water restricted to the inactive period; and (d) return to baseline. Time series analyses of nocturnal ONEs in baseline revealed a strong circadian rhythm and weaker ultradian rhythms with periods between 2 and 6 h. Analyses of FEs, though, revealed a general absence of circadian rhythms but strong ultradian rhythms with periods similar to those observed in ONEs. When food and water were restricted to the inactive part of the cycle, ONEs showed no change in frequency, but a decrease in average duration and changes in ultradian periodicities. The results indicated control of daily episodes of behavior in rats by ultradian oscillators that are weaker and more variable in affect than those found in voles, but similar in period.