Over the course of a normal 24 hours, the environment of, and the demands placed upon, cells/organs/organisms fluctuate dramatically. Organisms have therefore evolved circadian clocks, intracellular molecular mechanisms that allow individual cells to perceive the time of day. In doing so, circadian clocks confer the selective advantage of anticipation, enabling both rapid and appropriate responses to environmental stimuli/stresses upon their onset. Research in my laboratory has focused primarily on identifying the roles of the circadian clock within the heart. Over the past 15 years we have accumulated evidence supporting the concept that the cardiomyocyte circadian clock temporally partitions cardiac processes, including transcription, ion homeostasis, signal transduction, and metabolism. In the latter case, we have revealed that the cardiomyocyte circadian clock directly regulates myocardial glucose, fatty acid, and protein metabolism, promoting oxidative metabolism during the active phase (thereby providing ATP in anticipation of increased contractility) and protein turnover during the sleep phase (thereby promoting growth and repair in anticipation of the subsequent active period). In addition, we have found that the cardiomyocyte circadian modulates responsiveness of the heart to physiologic stimuli (e.g., insulin) and pathologic stresses (e.g., ischemia/reperfusion) in a time-of-day-dependent manner, thus synchronizing the heart with its environment.