Introduction: Recent global trends towards work intensification and digitalization, together with increasingly prevalent remote work modalities and cultures of long working hours, are posing emerging threats to employee health and well-being. Particularly, the impact of technology-assisted supplementary work (TASW) - i.e., tendency to perform work tasks with digital devices after regular work hours - on post-work recovery and sleep is increasingly acknowledged. Yet, while TASW has been consistently associated with poor sleep quality, inconsistent evidence has been provided on its relationship with sleep duration, with work-related perseverative cognitions (i.e., affective rumination and problem-solving pondering) having been proposed as possible intervening mechanisms. Purpose: To investigate the role of work-related perseverative cognitions as mediators of the relationship between TASW and sleep quality and quantity. Method: A cross-sectional survey (funded by MUR - Complementary National Plan PNC-I.1 PNC0000002 DARE - Digital Lifelong Prevention, CUP: B53C22006440001) was conducted with a representative sample of Italian workers (n = 1,585; 46% women; 52% remote/hybrid workers; mean age = 46.1 years) that reported working 18+ hours/week and using work-related devices at their home (e.g., smartphones and laptops). WLSMV-based structural equation modeling was used to test the study hypotheses. Results: Affective rumination and problem-solving pondering fully mediated the relationship between TASW and sleep quality, whereas only the latter mediated the relationship between TASW and sleep duration. The potential moderating roles of remote work and chronotype are also explored. Conclusions: Whereas problem-solving pondering is often considered a "positive" form of rumination that improves creativity without harm, our study highlights that it can be not only detrimental for both sleep quality and quantity, but also one of the main mechanisms through which digital overwork impairs overall sleep outcomes. Practical implications for human resources and work-life boundary management are discussed.