The circadian timing system in mammals: two pacemakers preside over many secondary oscillators

There is considerable evidence to indicate that circadian timekeeping in mammals is accomplished by a multioscillator system. Circadian rhythms in physiological variables may under certain circumstances adopt different periodicities within the same animal, and many mammalian tissues have been shown capable of generating persisting circadian rhythms in vitro. However, as yet only one circadian pacemaker has been identified unequivocally: the suprachiasmatic nuclei (SCN) in the anterior hypothalamus. This paper presents evidence that mammals possess another major circadian pacemaking system that is normally coupled to the SCN but that can function independently. Each of these major pacemakers presides over a considerable number of secondary oscillators located in diverse tissues.