of chest pain in acute myocardial infarction . Circadian variation in the frequency of onset

The time of onset of chest pain was studied prospectively in 1154 consecutive patients admitted to a coronary care unit with myocardial infarction during a five year period. Statistical analysis confirmed a previous finding in a retrospective study of a bimodal frequency distribution with peaks in the time of onset of chest pain between 2330 and 0030 hours and between 0630 and 0830 hours. Many cardiac disorders' including sudden death,2 angina,3 and asymptomatic myocardial ischaemia45 are more frequent at the normal waking time. Studies have shown that the onset of acute myocardial infarction also often occurs at this time.' However, in a retrospective study of the time of onset of chest pain in myocardial infarction7 our data also suggested a bimodal distribution, with a secondary peak around midnight. Though several studies found convincing evidence for a second, late evening peak6 II others have not found such a pronounced midnight peak. We have, therefore, completed a five year prospective study of the time of onset of chest pain in 1154 patients admitted to a coronary care unit with a myocardial infarction. Coronary Care Unit, Leicester General Hospital, Leicester D R Thompson N I Jowett J E F Pohl School of Mathematics and Computing, Leeds Polytechnic, Leeds TW Sutton Correspondence to Dr David R Thompson, Coronary Care Unit, Leicester General Hospital, Leicester LE5 4PW. Accepted for publication 6 December 1990 Patients and methods