Thirty-eight patients with lone paroxysmal atrial fibrillation (PAF) were monitored during spontaneous sleep. The starts of 111 episodes of atrial fibrillation (AF) occurred in 12 patients. Statistical analysis failed to disclose significant differences in the number of starts per hour between wakefulness and the different sleep stages. Heart rate (HR) in the 60 seconds prior to a PAF episode did not show significant changes with respect to fragments of recording taken away from PAF episodes. As expected, spectral analysis of HR variability during sleep (basal values) showed a reduction of low frequency/high frequency (LF/HF) ratio during slow sleep, while the ratio returned to values similar to wake levels during REM sleep. In the 2 minutes preceding a PAF episode the LF/HF ratio of the spectral power was more unstable with a clear-cut prevalence of sympathetic activity both in wake and in different sleep stages. A shift toward parasympathetic activation was never seen before the start of any of the 17 PAF episodes recorded during sleep.