While functional roles for biological clocks have been demonstrated in organisms throughout phylogeny, the adaptive advantages of circadian organization per se are largely matters of conjecture. It is generally accepted, though without direct experimental evidence, that organisms derive primary benefits from the temporal organization of their physiology and behavior, as well as from the anticipation of daily changes in their environment and their own fluctuating physiological requirements. However, the consequences of circadian dysfunction that might demonstrate a primary adaptive advantage and explain the natural origins and apparent ubiquity of circadian systems have not been documented. The authors report that longevity in hamsters is decreased with a noninvasive disruption of rhythmicity and is increased in older animals given suprachiasmatic implants that restore higher amplitude rhythms. The results substantiate the importance of the temporal organization of physiology and behavior provided by the circadian clock to the health and longevity of an organism.