The heart, like most mammalian organs, is influenced by circadian patterns. The suprachiasmatic nucleus in the hypothalamus has a key role in this influence, via various neurohumoral factors, particularly the autonomic nervous system. In addition, a local cardiac peripheral clock might drive a circadian rhythm related to the expression of ion channels. Several myocardial functions are influenced by these circadian cycles including activity/rest, regeneration, nutrient storage, growth, and myocardial repair. Numerous circadian genes have been identified in basic studies, and both biological factors and environmental features (including epigenetic) influence the human circadian rhythm. A normal circadian rhythm is important to maintain a normal heart rhythm and circadian rhythm disturbances can predispose to the development of cardiac arrhythmias. The normal heart rate presents a daily variability with a morning peak and nocturnal bradycardization. Ventricular arrhythmias and sudden death are more likely to occur in the morning after waking, while atrial fibrillation and heart blocks most commonly occur at night. Drugs such as beta-blockers might modify the chronobiology of some of these arrhythmias. On the other hand, drugs that influence circadian rhythm, like the circadian hormone melatonin, have demonstrated pleiotropic properties and show promising results as antiarrhythmics. This review is focused on the current understanding of the basic mechanism and clinical implications of the association circadian rhythms-cardiac arrhythmias/sudden death. The close relationship between circadian patterns and arrhythmias may provide us with the possibility of novel interventions to decrease the arrhythmic risk in some patients.