In the present work, f-wave morphology is characterized by principal component analysis and a novel temporal parameter defined by the cumulative normalized variance of the 3 largest principal components (r3). The 7-day behavior of persistent atrial fibrillation (AF) was studied in 9 patients using r3, AF frequency, and sample entropy (SampEn). Detection of circadian rhythms depended on the parameter considered: rhythms were found in 6 (r3, SampEn) and 5 (AF frequency) patients, but interestingly not always in the same patients. Two patients had significant circadian rhythm in all parameters. When a circadian rhythm was significantly present in 7 days, it was usually only significantly present in some of the 24-h segments. It is concluded that detailed AF characterization can be achieved with complementary parameters.