BACKGROUND: Sleep disturbance in bipolar disorder can be both a risk factor and symptom of mood episodes. However, the associations among sleep and clinical characteristics, function, and quality of life in bipolar disorder have not been fully investigated. METHODS: The prevalence of sleep disturbance, duration, and variability, as well as their associations with mood, function, and quality of life, was determined from 2024 bipolar patients enrolled in the National Institute of Mental Health Systematic Treatment Enhancement Program for Bipolar Disorder (STEP-BD). RESULTS: Analyses indicated that 32% of patients were classified as short sleepers, 38% normal sleepers, and 23% long sleepers. Overall, short sleepers demonstrated greater mood elevation, earlier age at onset, and longer illness duration compared to both normal and long sleepers. Both short and long sleepers had greater depressive symptoms, poorer life functioning, and quality of life compared to normal sleepers. DISCUSSION: Short sleep duration in bipolar disorder was associated with a more severe symptom presentation, whereas both short and long sleep duration are associated with poorer function and quality of life compared to normal sleep duration. Sleep disturbance could be a trait marker of bipolar disorder, though longitudinal assessments are warranted to assess potential causal relations and the longer-term implications of sleep disturbance in bipolar disorder.