Little to no research has evaluated factors that explain the manifestation and maintenance of bipolar disorder-anxiety disorder (BD-AD) comorbidity. The literature has shown that disruption of social and circadian rhythms is associated with mood episode onset. This association is especially pronounced among individuals who have a sensitive behavioral approach system (BAS). Inasmuch as anxiety sensitivity and trait anxiety have been associated both with BD and social rhythm disruption, it is worth examining whether anxiety sensitivity and trait anxiety confer risk for mood episode onset. The aims of this project were to: 1) evaluate trait anxiety and anxiety sensitivity as predictors of social rhythm disruption and BD-AD comorbidity, 2) examine social rhythm disruption (SRD) as a mediator of the association between trait anxiety and anxiety sensitivity and BD-AD comorbidity status, and 3) explore behavioral approach system sensitivity in these processes as contributing to the vulnerability to BD-AD comorbidity. A sample of 156 young adults participated in a multi-wave study in which they completed diagnostic interviews, symptom measures, and life event interviews which assessed the occurrence of positive and negative life events and the degree of SRD following these events every six months. Partial support for the hypotheses was found. Initial anxiety sensitivity, but not trait anxiety, positively predicted SRD for rewarding life events and follow-up bipolar symptoms. Additionally, SRD following positive life events predicted increases in depressive symptoms, but not hypomanic symptoms. SRD mediated the relationship between anxiety sensitivity and depressive symptoms. Furthermore, this relationship was stronger for healthy controls than for those with a bipolar disorder (BD) diagnosis or at-risk for developing BD. Moreover, individuals with a comorbid BD-AD diagnosis tended to have greater social rhythm disruption following negative life events than BD only or healthy individuals. Unexpectedly, individuals with comorbid BD-AD did not exhibit greater anxiety sensitivity or trait anxiety. Overall, the results suggest that anxiety sensitivity is a factor that may help to identify individuals who are vulnerable to bipolar symptoms. Furthermore, SRD is a mechanism that may partially explain this relationship. / Psychology