Restricted daytime feeding attenuates reentrainment of the circadian melatonin rhythm after an 8-h phase advance of the light-dark cycle

It is well established that in the absence of photic cues, the circadian rh ythms of rodents can be readily phase-shifted and entrained by various nonp hotic stimuli that induce increased levels of locomotor activity (i.e., ben zodiazepines, a new running wheel, and Limited food access). In the presenc e of an entraining light-dark (LD) cycle, however, the entraining effects o f nonphotic stimuli on (parts of) the circadian oscillator are far less cle ar. Yet, an interesting finding is that appropriately timed exercise after a phase shift can accelerate the entrainment of circadian rhythms to the ne w LD cycle in both rodents and humans. The present study investigated wheth er restricted daytime feeding (RF) (1) induces a phase shift of the melaton in rhythm under entrained LD conditions and (2) accelerates resynchronizati on of circadian rhythms after an 8-h phase advance. Animals were adapted to RF with 2-h food access at the projected time of the new dark onset. Befor e and at several time points after the 8-h phase advance, nocturnal melaton in profiles were measured in RF animals and animals on ad Libitum feeding ( AL). In LD-entrained conditions, RF did not cause any significant changes i n the nocturnal melatonin profile as compared to AL. Unexpectedly, after th e 8-h phase advance, RF animals resynchronized more slowly to the new LD cy cle than AL animals. These results indicate that prior entrainment to a non photic stimulus such as RF may "phase lock" the circadian oscillator and in that way hinder resynchronization after a phase shift.