Aspirin AM or PM: Effect on Circadian Rhythm of Platelet Reactivity

Cardiovascular events are a leading cause of mortality and morbidity in western countries. In the European Union, 47% of total mortality is caused by cardiovascular disease2. Aspirin is a cornerstone in the secondary prevention of cardiovascular disease because of its inhibitory effects on platelet aggregation. It reduces the risk of recurrent cardiovascular events with about a quarter3. Although not supported by evidence, aspirin is usually taken in the morning, but it may be more beneficial to take aspirin at bedtime instead of on awakening. It has been convincingly shown that platelet activity follows a circadian rhythm, with a peak of platelet reactivity in the morning4-8. This might in part explain the increase in cardiovascular events in the early morning, with the highest incidence between 6 and 12 AM1. Since platelet reactivity follows a circadian rhythm, the timing of aspirin intake may influence its inhibitory effect on platelets. Due to its short half-life, aspirin only inhibits platelets which are present at the time of intake. New platelets are released at a rate of 10%/day, predominantly during the night9. Because they are more reactive and not inhibited by aspirin taken in the preceding morning, these young platelets contribute to the morning peak of platelet reactivity10, 11. It has been argued that intake of aspirin at bedtime could better prevent the early morning increase in platelet reactivity than intake on awakening, assuming that intake on awakening would be too late to prevent this morning peak in platelet reactivity12. Additionally, a recent study showed significant recovery of platelet aggregation after 24 hours in patients using low-dose aspirin on a daily basis13. This supports the hypothesis that aspirin intake at bedtime could be beneficial in reducing the morning peak of platelet reactivity, thereby possibly also reducing the incidence of arterial thrombotic events in the morning. However, this has never been studied before.