Entrainment by red light of running activity and ovulation rhythms of rats

Controversy exists regarding whether or not the circadian rhythms of nocturnal rodents can be entrained by red light. Female albino rats that were free-running in continuous darkness (DD) were exposed to 2 h of red light (> 650 nm) near the beginning (dusk signals) or the end (dawn signals) of their active 12-h period of running. Red dawn signals advanced and red dusk signals delayed the onset of running on subsequent days. Because the altered onsets of running persisted in DD, the red light had produced a true entrainment of the circadian rhythm of running activity. The fact that the time of ovulation was similarly shifted by red light suggests that the circadian rhythm of luteinizing hormone secretion was also entrained by red light.