Cardiologists have analyzed daily patterns in the incidence of sudden cardiac death to identify environmental, behavioral, and physiological factors that trigger fatal arrhythmias. Recent studies have indicated an overall increase in sudden cardiac arrest during daytime hours when the frequency of arrhythmogenic triggers is highest. The risk of fatal arrhythmias arises from the interaction between these triggers such as elevated sympathetic signaling, catecholamine levels, heart rate, afterload, and platelet aggregation and the susceptibility of the heart (myocardial substrate) to them. A healthy myocardial substrate has structural and functional properties that protect against arrhythmias. However, individuals with cardiovascular disease often exhibit structural and electrophysiological alterations in the myocardial substrate that predispose them to sustained lethal arrhythmias. This review focuses on how day-night and circadian rhythms, both extrinsic and intrinsic, influence the protective properties of the myocardial substrate. Specifically, it explores recent advances in the temporal regulation of ion channel gene transcription, drawing on data from comprehensive bioinformatics resources (CircaDB, CircaAge, and CircaMET) and recent RNA sequencing studies. We also examine potential mechanisms underlying the temporal regulation of mRNA expression and the challenges in linking rhythmic mRNA expression to corresponding changes in protein levels. As chronobiological research in cardiology progresses, we anticipate the development of novel therapeutic strategies to enhance the protective properties of the myocardial substrate to reduce the risk of fatal arrhythmias and sudden cardiac arrest.