BACKGROUND: Few investigators have examined the hemodynamic status related to circadian and sleep-related cardiovascular events in critically ill patients. OBJECTIVE: To describe the relationships among nocturnal angina, myocardial infarction, and sleep, and to describe the cardiac rhythm, heart rate, and frequency of premature supraventricular and ventricular contractions during the sleep of critical care patients with cardiovascular disease. METHODS: A descriptive survey was done on nine patients in the medical intensive care unit of a large medical center. All subjects had cardiovascular disease and were studied during one night of sleep. RESULTS: There were no incidences of chest pain during the study and no changes in baseline cardiac rhythm associated with sleep staging. There was a significant statistical difference in mean heart rate among waking, stage 1 nonrapid-eye-movement sleep, and stage 2 nonrapid-eye-movement sleep; a lower heart rate was observed in stage 2 nonrapid-eye-movement sleep. The mean heart rate decreased by 3% to 4% from the waking state to sleep. No clinical or statistically significant differences in the frequency of premature supraventricular and ventricular contractions between sleep stages or sleeping and waking were found. CONCLUSIONS: The results of this pilot investigation do not suggest that dysrhythmia occurs more frequently during certain sleep stages or during the sleep state in critical care unit patients with cardiovascular disease. The mean heart rate slowed by 3% to 4% during rapid-eye-movement and nonrapid-eye-movement sleep, but decreases in heart rate were not as great as those noted in normal, healthy subjects.