Reentrainment of the circadian pacemaker through three distinct stages

Abstract Circadian rhythms are endogenously generated by a central pace-maker and are synchronized to the environmental LD cycle. The rhythms can be resynchronized, or reentrained, after a shift of the LD cycle, as in traveling across time zones. The authors have performed high-resolution mapping of the pace-maker to analyze the reentrainment process using rat pineal melatonin onset (MTon) and melatonin offset (MToff) rhythms as markers. Following LD (12:12) delays of 3, 6, and 12 h, MTon was phase locked immediately, whereas MToff shifted rapidly during the initial 1 through 3 cycles. In all animals, the MToff shifted beyond their expected phase positions in the new LD cycle, which resulted in a transient expansion of melatonin secretion duration for several cycles. It took MToff only 1, 2, or 3 cycles to complete most of the required phase shifts after 3, 6, or 12 h of the LD cycle delays, respectively. However, the final sta-bilization of phase relationships of both MTon and MToff required at least 6 cycles for rats experiencing a 3-h LD delay and much longer for the rest. These results reaffirmed the notion that both onset and offset phases of melatonin rhythms ar