Myocardial infarction shows a circadian pattern with a maximum in the early morning hours. In patients with sleep-related breathing disorders (SRBD), it is assumed that apnea-associated changes of hemodynamics, blood gases, and rheology lead to a higher frequency of myocardial infarction during sleep. This investigation analyzes the circadian pattern of myocardial infarction in patients with and without SRBD. Within a time period of 20 months, 89 male patients with acute myocardial infarction were consecutively admitted to the intensive care unit. A nocturnal long-term registration of oxygen saturation, heart rate, breathing sounds, and body position by means of a 4-channel recording system (MESAM IV) was carried out in 59 of the 89 patients 6 to 10 days (evaluation I) and in 43 of 59 patients 22 to 28 days after infarction (evaluation II). Sleep apnea with a respiratory-disturbance-index (RDI > or = 10/h was found in 44.1/39.5% of the patients (evaluation I/II). In 22% of the patients, time of infarction was during a sleeping period. Patients with myocardial infarction during sleep had a clearly higher RDI in comparison to patients with a myocardial infarction during wakefulness (evaluation I: 22.7 versus 9.4/h; p = 0.08; evaluation II: 20.3 versus 7.3; p < 0.05). 53.6% of all myocardial infarctions occurred during the time period 5:00-11:00 a.m. Investigations in a larger number of patients are necessary to confirm these results as well as the relevance of sleep apnea as a cardiovascular risk factor.