CLOCK and Ventricular arrhythmia: This sympathetic activation, along with concurrent increases in HR, BP, and myocardial oxygen demand, are thought to increase the risk of ventricular arrhythmias (VA) by triggering abnormalities in intracellular calcium handling and action potential repolarization.67 Another actively studied mechanism underlying the susceptibility of VA in the morning hours is the circadian clock dependent modulation of electrical activity driven by functional changes of cardiac ion channels and extrinsic autonomic receptor mediated mechanisms (reviewed by Bernardi et al.68).