Objective : Dysfunctional cognitions related to sleep play a major role in insomnia but also in nightmares. Moreover, they are closely related to anxiety. To our knowledge, no study has probed the impact of non-constructive ruminations on these dimensions in their harmful interplay with sleep. The aim of this study is to provide new insights into the processes underlying the dysfunctional cognitions-insomnia relationship. Method : Four hundred twenty nine French participants completed an anonymous online survey using Qualtrics® software. For the assessment of variables, we used the Mini Cambridge-Exeter Repetitive Thought Scale, the Nightmare Distress Questionnaire, the Beck Anxiety Inventory and the Sleep Condition Indicator. The frequency of nightmares was assessed subjectively via an item. Participants were divided into two groups according to their score on the insomnia assessment: good sleepers and poor sleepers. Result : Anxiety was found to be a common mediator of the relationship between dysfunctional beliefs and attitudes toward sleep and insomnia between good (20.8%) and poor sleepers (24.6%). However, for poor sleepers, nightmare frequency (15.2%) and non-constructive ruminations (16.6%) emerged as mediators of this relationship. Conclusion : The results gathered through this study bring interesting perspectives regarding the theoretical and etiological conceptualization of insomnia. We showed a positive association between dysfunctional beliefs and attitudes towards sleep and non-constructive ruminations in their contributory role to insomnia.