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This chapter shows that circadian rhythms have a tendency to become less robust with increasing age, i.e. they generally exhibit decreases in amplitude and less stability. Some obvious consequences of such reductions in rhythm amplitude are fragmentation of rhythms, complete loss of temporal order and structure, loss of stability of entrainment, and responsiveness to zeitgebers. Other consequences include changes in clock period and its stability, and inappropriate phase relationships among behavioral and metabolic oscillations. Age-related decreases in the amplitude of circadian rhythms in humans and other mammals have been linked to a deterioration of rhythmic behaviors such as those seen in locomotor activity, feeding and drinking. Anatomical and electrophysiological studies have shown that age-related changes occur within the biological clocks of mammals including humans. There is increasing evidence for age-related changes in the structure and neurochemistry of the suprachiasmatic nucleus (SCN), including alterations in cells producing vasopressin.