Circadian variation in circulating platelet aggregates.

Acute coronary events are more likely to occur in the morning and it has recently been shown that platelet aggregability increases significantly then. To determine whether circulating platelet aggregates show such a circadian variation, we measured in vivo platelet aggregation in atherosclerotic patients and healthy subjects at three-hour intervals throughout a 24-hour period. We found a significant variation in circulating platelet aggregates, the highest being at 6 a.m. and the lowest at 9 a.m. We suggest that it may be important to consider the duration of action of anti-aggregating drugs and timing their administration with respect to circadian variation in circulating platelet aggregates.