Background—Myocardial infarction, stroke, and sudden death undergo diurnal variation. Although genes relevant to hemostasis and vascular integrity undergo circadian oscillation, the role of the molecular clock in thrombotic events remains to be established.

Methods and Results—A diurnal variation in the time to thrombotic vascular occlusion (TTVO) subsequent to a photochemical injury was observed in wild-type mice: TTVO varied from 24.6±2.7 minutes at zeitgeber time (ZT) 2 to 40.3±4.3 minutes at ZT8, 24.3±2.3 minutes at ZT14, and 31.0±4.4 minutes at ZT20. This pattern was disrupted or altered when core clock genes—BMAL1, CLOCK, and NPAS2—were mutated or deleted. Mutation of CLOCK abolished the diurnal variation in TTVO, whereas deletion of NPAS2 altered its temporal pattern. NPAS2 deletion prolonged TTVO and reduced blood pressure irrespective of clock time. Global BMAL1 deletion shortened TTVO at ZT8, and the diurnal variation in TTVO, but not in systemic blood pressure, was disrupted in mice in which BMAL1 had been selectively deleted in endothelium.

Conclusions—Key components of the molecular clock regulate the response to a thrombogenic stimulus in vivo. Such a phenomenon may interact with environmental variables, and together with the influence of these genes on blood pressure may contribute to the diurnal variation in cardiovascular events observed in humans.