Many studies have shown that the risk of experiencing a myocardial infarction (MI) is increased during the first hours of the morning. Sleep apnea syndrome (SAS) is associated with an enhanced adrenergic activity, prolonged a few hours after awakening. We aimed at assessing whether sleep breathing disorders could be a culprit for the morning excess rate of MI. We studied 40 middle-aged men admitted for an acute MI. An overnight polysomnographic study was performed 37.4 +/- 9.4 days after the MI. The prevalence of SAS was high (30%). The prevalence of SAS was significantly higher in patients with the MI onset during the morning. The circadian pattern was significantly different in patients with or without SAS: those with SAS presented an important peak of MI onset during the period between 06.00 and 11.59 h. None of them had their MI during the period between 24.00 and 05.59 h. This different nyctohemeral pattern underlines the potential role of sleep breathing disorders as a trigger of MI.