The suprachiazmatic nucleus (SCN) is a circadian pacemaker structure that drives myriad behavioral and physiological rhythms in mammals. Surgical destruction of the SCN abolishes most circadian rhythms; however, some rhythms persist including mitosis in cornea and gut and disk shedding in the retina. Circadian rhythms in wheel-running behavior, absent following complete SCN lesions, reappear when methamphetamine is provided chronically to SCN-lesioned rats. Furthermore, SCN-lesioned rats express a rhythm in food-anticipatory activity when exposed to restricted feeding and later fasted. Collectively, these observations reveal the existence of extra-SCN circadian oscillators. More recent results suggest that they may be present in most mammalian cells and tissues.
Age-related changes in the mammalian circadian timing system have been studied in rodents and in humans. In several species of mammals, aging affects the amplitude and phase of circadian rhythms. The period of the circadian locomotor activity rhythm is shortened in aged rats and in aged hamsters by most reports; however, two studies of hamsters failed to reveal age-related changes in period. The effect of aging on the free-running period in mice has not been consistent. Similarly, in humans there have been conflicting results. An early study reported that the free-running period changed with age; however, a recent, more comprehensive study reveals that the period of the sleep-wake rhythm is essentially identical in young and elderly subjects. Aging seems to affect rhythms in some but not in all tissues and may act primarily on interactions among circadian oscillators, perhaps attenuating the ability of the SCN to drive damped oscillators in the periphery.