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Introduction : Abnormalities in biological rhythm have been received attentions as an endophenotype in bipolar disorder. In previous study, authors have reported that seasonality was prominently observed in bipolar patients. This study aimed to investigate whether seasonality and sleep-wake cycle characteristics were more frequently observed in unaffected siblings of bipolar patients. Method : Subjects were 102 clinically stable patients with BD, 30 of their unaffected siblings and 201 healthy controls. Seasonality was measured using the Seasonal Pattern Assessment Questionnaire (SPAQ). Circadian preference and sleep irregularity were assessed using the Composite Scale of Morningness and the Sleep Timing Questionnaire, respectively. Results : The patient group showed higher global seasonality scores on the SPAQ compared to both sibling and control groups. There was no difference between sibling and control groups. The circadian-preference did not significantly differ among three subject groups. Compared to the control group, both patient and sibling groups showed significantly higher irregularity in goodnight time during weekdays. Conclusion : Seasonality and irregularity in sleep-wake cycle could be regarded as lifetime traits related to BD. Irregularity in weekday sleep cycle was also prominent in the unaffected siblings of the patients for which further investigation as a behavioral phenotype related to the vulnerability to BD is warranted. (J of Kor Soc for Dep and Bip Disorders 2011;9:70-77)