Over the past decade, a large body of literature has demonstrated that disruptions of the endogenous circadian clock, either environmental or genetic, lead to metabolic dysfunctions associated with obesity, diabetes, and other metabolic disorders. The phrase, "It is not only what you eat and how much you eat, but also when you eat" sends a simple message about circadian timing and body weight regulation. Getting this message out to clinicians and patients, while at the same time the elucidation of neuroendocrine, molecular, and genetic mechanisms underlying this phrase makes it very likely that the circadian impact on lifestyle and clinical practice could be soon adopted for improving overall human health. In this review, we discuss findings from animal models as well as epidemiological and clinical studies in humans, which collectively promote the awareness of the role of circadian clock in metabolic functions and dysfunctions.