There is now reason to speculate that disruption of circadian rhythms of physiology and behavior may have broader implications for human health. A long history of clinical epidemiology in humans demonstrates an increased incidence of obesity, cardiovascular disease and cancer among shift workers. Clues from studies on the molecular genetics of circadian clock genes may offer insight into the molecular mechanisms underlying the circadian variation of metabolic coordination. A better understanding of the impact of circadian gene networks on nutrient balance at the molecular, cellular, and system levels promises to shed light on the emerging association between disorders of diabetes, obesity, sleep, and circadian timing.