Aims To determine the circadian rhythm of paroxysmal atrial fibrillation in a very large outpatient population.

Methods and Results We reviewed all emergency telephone calls received in Shahal (a medical service covering 44000 subscribers), from 1987 to 1997. Patients were included if new-onset atrial fibrillation was recorded. During this study period, 9989 episodes of paroxysmal atrial fibrillation were recorded. The time of onset was not uniformly distributed throughout the 24h period. Instead, the distribution of arrhythmic episodes showed a double peak, with a significant increase in the number of episodes in the morning and a second rise in the evening (P<0·001). A non-uniform weekly distribution of events was also noted, with substantially fewer episodes on Saturdays (P<0·001). Finally, more arrhythmias occurred during the last months of each year (P<0·001).

Conclusions The onset of paroxysmal atrial fibrillation does not occur randomly. The large patient population in the present study suggests that the circadian rhythm of paroxysmal atrial fibrillation is similar to that described for other cardiovascular diseases, with clustering of events in the morning and (to a lesser degree) late in the evening. Weekly and yearly circadian patterns are also prominent.