Technical Abstract: The heart is challenged by a plethora of extracellular stimuli over the course of a normal day, each of which distinctly influences myocardial contractile function. It is therefore not surprising that myocardial metabolism also oscillates in a time-of-day dependent manner. What is becoming increasingly apparent is that the heart exhibits diurnal variations in its intrinsic properties, including responsiveness to extracellular stimuli. This article summarizes our current knowledge regarding the mechanism(s) mediating diurnal variations in myocardial metabolism. Particular attention is focused towards the intramyocardial circadian clock, a cell autonomous molecular mechanism that appears to regulate myocardial metabolism both directly (e.g., triglyceride and glycogen metabolism) and indirectly (through modulation of the responsiveness of the myocardium to workload, insulin, and fatty acids). In doing so, the circadian clock within the cardiomyocyte allows the heart to anticipate environmental stimuli (such as changes in workload, feeding status) prior to their onset. This synchronization between the myocardium and its environment is enhanced by regular feeding schedules. Conversely, loss of synchronization may occur through disruption of the circadian clock and/or diurnal variations in neurohumoral factors (as observed during diabetes mellitus). Here, we discuss the possibility that loss of synchronization between the heart and its environment predisposes the heart to metabolic maladaptation and subsequent myocardial contractile dysfunction.