It is almost common sense that work stress leads to sleep impairment, but the question of how work-related stressors impair employee sleep remains open. This study focuses on the role of rumination as the underlying mechanism for sleep impairment. Specifically, the authors contribute to recent research differentiating affective rumination from problem-solving pondering and examine the impact of both forms of rumination on the stressor–sleep relationship. Following theories of rumination and the Zeigarnik effect, they focus on unfinished tasks as a key onset for rumination. Unfinished tasks have received much research attention in the memory context but have been neglected as a stressor that can impact recovery. Drawing on theory, differential indirect links between unfinished tasks and sleep through affective rumination versus problem-solving pondering are examined. Further, the number of unfinished tasks extending over a 3-month period may impair employee sleep more than unfinished tasks within the acute phase. In this study, intraindividual links in a diary study supplemented by depicting between-person effects of unfinished tasks over a period of 3 months are examined. The authors matched 357 Friday and Monday observations over a 12-week interval for 59 employees. The results of the multilevel analysis suggest that the within-person relationship between unfinished tasks and sleep is mediated by affective rumination. Although problem-solving pondering was negatively related to sleep impairment, the indirect effect was not significant. Finally, beyond the acute effect, the authors found higher levels of unfinished tasks over 3 months are related to increased sleep impairment on the weekend.