The risk of acute cardiovascular events is highest in the morning hours and platelet activity is also highest during this time. A small, randomised, cross-over trial in healthy patients in the Netherlands was conducted to evaluate the effect of the time of aspirin dose on platelet activity. 

At the end of the study period COX-1- dependent and COX-1-independent platelet activity was measured. COX-1-dependent platelet activity was decreased by intake of aspirin at bedtime compared with on awakening whereas COX-1-independent activity was unaffected by the timing of the aspirin dose.

Further trials are needed to determine whether taking aspirin at bedtime reduces the risk of cardiovascular events during the high risk morning hours.

[Bonten, TN et al; Thromb Haemost 11 Sep 2014; 112 (6)]