- Rumination and dissociation are considered maladaptive cognitive-emotional responses to distress, which prevent its proper processing. The goal of the study was to explore the relationship between rumination and dissociation and possible mechanisms responsible for it. We hypothesized that rumination may exert influence on dissociation via four possible mediators: distress, perceived mental control, repetitive cognitive style, and poor sleep quality. Study 1 was conducted on a sample of 93 undergraduate students and pointed to a central role of sleep in this relationship. Study 2 included 218 undergraduate students and aimed to: (a) replicate the results of Study 1 and (b) focus on the sleep factor and explore the role of bedtime strategies for dealing with negative thoughts. Both studies point to sleep quality as the central factor responsible for the positive link between rumination and dissociation. The repetitiveness and the negative valence of rumination predict poor sleep quality, which in turn predicts higher levels of dissociative experiences. We also found that ruminative individuals who cope with intrusive thoughts at bedtime who used maladaptive and rigid strategies (aggressive suppression, behavior distraction and worry) slept poorly and reported more dissociative symptoms. We conclude that rumination and dissociation are linked through the disturbed cycle of wake and sleep states and impaired flexibility in transitioning between them. Implications for clinicians and researchers are discussed.