Seasonal variation in the occurrence of atrial fibrillation (AF) has been documented, yet precise mechanisms and factors underlying the phenomenon remain unclear. We have previously observed the decrease in the number of AF paroxysms between May and August, when sunshine levels were highest. The objective of the present study was, in turn, to determine whether sunshine affects the incidence of AF episodes. Participants were 1,475 Caucasian subjects (mean age: 68.2 years) diagnosed with AF paroxysms, admitted to the Intensive Cardiac Care Unit (ICCU) between January 1, 2005 and December 31, 2008; 805 were women and 670 were men (mean age: 69.2 and 66.2, respectively). The average incidence of AF episodes was higher among female subjects, with 16.8 cases per month, compared to male subjects with 14.0 cases per month. Pearson's correlation coefficient (r) was used to find a relationship between monthly sums of sunshine duration and AF paroxysms. This relationship for women was clearly inversely proportional (r = -0.499); namely, most AF episodes were recorded from December to March, when sunshine levels were lowest. In contrast, there was no noticeable association in male patients between the occurrence of AF paroxysms and effective sunshine (r = -0.126). In conclusion, unlike in men, a marked, statistically confirmed relationship between AF episodes and effective sunshine was observed in women. Thus, sunshine may have a protective effect against AF paroxysms for women. Our findings may provide the basic information concerning the influence of environmental factors on human wellbeing and contribute to management of AF.