<p>Few studies have examined the spillover effects of workload on off-work rumination and psychological detachment, and more precisely the mechanisms leading to it. Drawing on Self-Determination Theory, we proposed that psychological need frustration explains how workload leads to work-related rumination (affective rumination and problem-solving pondering) and difficulties in psychologically detaching from work over time. More specifically, using a two-wave panel design and a cross-lagged statistical approach, this study investigated the temporal relationships among these variables and the mediating role of need frustration (autonomy, competence, and relatedness). This study was conducted among a sample of 282 nurses who completed an online questionnaire at two time points over a 12-month period. Cross-lagged analyses revealed that, controlling for baseline effects, T1 workload positively predicted T2 need frustration (both autonomy and competence), T2 affective rumination, T2 problem-solving pondering, and negatively predicted T2 psychological detachment. Furthermore, the relationship between T1 workload and T2 affective rumination was mediated by T1 competence need frustration. In sum, these results highlight that, over time, workload undermines employees’ competence needs, which fosters recurrent negative thoughts and feelings about work (i.e., affective rumination). Therefore, organisational efforts to reduce employees’ workload and support their psychological needs are advised to decrease off-work work-related rumination and promote psychological detachment.</p>