Objectives Sleep-wake disturbances, such as sleep irregularity, are common in bipolar disorder. Early studies suggest that sleep irregularity is associated with mood symptoms in bipolar disorder, but little research has been conducted to identify other correlates of sleep irregularity. We investigated the relationship between sleep irregularity and sleep quality, social rhythms, eveningness, sleep-related cognitions and behaviors, and past and future mood episodes in 84 patients with inter-episode bipolar I or II disorder. Methods This is a retrospective and prospective, naturalistic follow-up study. The Expanded Consensus Sleep Diary, Pittsburgh Sleep Quality Index (PSQI), Social Rhythm Metric (SRM-II-5), Composite Scale of Morningness (CSM), Dysfunctional Beliefs and Attitudes About Sleep Scale (DBAS-16), and Sleep Hygiene Practice Scale (SHPS) were administered. The Square Successive Difference (SSD), derived from a week-long sleep diary, was used as an index of sleep irregularity. Multilevel modeling analysis, which adjusts for biases in parameter estimates, was used to minimize the impact of missing data. Bonferroni correction was performed to account for multiple testing. Results Higher SSD scores of sleep diary variables were significantly associated with higher PSQI, SRM-II-5, DBAS-16, and SHPS scores. Irregularity in total sleep time was related to more depressive episodes in the past 5 years (p =.002), while irregularity in wake after sleep onset predicted the onset of depressive episodes over the next 2 years (p =.002). Conclusion Sleep irregularity was associated with poor sleep quality, irregular social rhythms, dysfunctional sleep-related cognitions and behaviors, and greater number of depressive episodes in bipolar disorder.