Various theories have been proposed to explain the onset of bipolar disorders (BD). Their development is likely influenced by a complex interplay of predisposing and risk factors, further complicated by the burden of the illness. Disturbances in circadian rhythms are recognized as significant contributors and often serve as key diagnostic criteria. Alterations in circadian rhythms are central to BD, with common manifestations including reduced amplitude of the activity–rest rhythm, phase shifts in sleep timing, and sleep fragmentation. These disturbances are also prevalent among individuals at high risk for BD. Therefore, sleep and circadian disturbances may act as early markers for BD in high-risk youth, making their monitoring essential for early intervention and improved prognosis. This chapter discusses the molecular mechanisms through which the circadian system influences mood and the manifestations of circadian disruptions in BD and high-risk individuals. Additionally, we examine distinct assessment methods for evaluating circadian and sleep disturbances and therapeutic strategies aimed at stabilizing circadian rhythms in BD.