Conceptual similarities between recent models of insomnia and emotional disorders suggest there may be common factors that underlie or maintain these difficulties. Maladaptive cognitive and behavioral processes similar to those described in connection with emotional disorders have been cited as key mechanisms in the maintenance of primary insomnia. Unfortunately, research on this potential overlap is lacking. The present study examined the relationship among anxiety sensitivity (AS), dysfunctional beliefs, fatigue, safety behaviors, and insomnia severity in 59 outpatients with anxiety and mood disorders. Key insomnia processes (dysfunctional beliefs, fatigue, safety behaviors) were all related to insomnia severity in the comorbid sample, although AS was not. However, as hypothesized, AS did moderate the relationship of both dysfunctional beliefs and fatigue with insomnia severity. The relationships between key insomnia processes and insomnia severity was strongest among individuals high in AS. Results support the hypothesis that common mechanisms are involved for insomnia and emotional disorders. AS might function as a mechanism for the maintenance of sleep disturbance in the context of anxiety and mood disorders, suggesting a promising avenue for future research.