PurposeGrounded in the conservation of resources theory, this study proposes the mechanisms and conditions under which customer mistreatment affects employee proactive behavior. This study focuses on insomnia as a mediating mechanism and resilience as a boundary condition for the indirect effect of customer mistreatment on employee proactive behavior via insomnia.Design/methodology/approachWe conducted a single survey in two phases, with a time lag of three weeks, among 302 frontline South Korean employees. The data collected from these two points were then analyzed using hierarchical regression analysis and the PROCESS macro.FindingsThe findings demonstrated that there was a significant negative relationship between customer mistreatment and employee proactive behavior, and this relationship was mediated by insomnia. Furthermore, the results revealed that resilience moderates both the effect of customer mistreatment on insomnia and the indirect effect of customer mistreatment on employee proactive behavior through insomnia.Research limitations/implicationsThis research primarily focuses on Korean frontline workers, potentially limiting cultural generalizability. The reliance on self-reported data may introduce common method bias. Future studies should diversify participant demographics and utilize multi-source feedback to validate findings. Grounded in the Conservation of Resource Theory, this study underscores the neglected linkage between customer mistreatment and proactive behavior, especially in frontline employees. We introduce insomnia as a pivotal mediator, deepening our understanding of why mistreatment dampens proactivity. Additionally, we spotlight the role of resilience, revealing its buffering effect against mistreatment’s adverse outcomes.Practical implicationsOrganizations should be aware of the detrimental effects of customer mistreatment on frontline employees, as it can hamper proactive behavior, primarily through the exacerbation of insomnia. Implementing resilience-training programs can be a proactive step, offering frontline staff tools to buffer against such negative outcomes. Managers are encouraged to recognize and address instances of customer mistreatment and prioritize employee well-being, which in turn can foster a more proactive and resilient workforce, enhancing organizational performance and customer satisfaction.Social implicationsThis study underscores the broader societal challenge of customer mistreatment in the service sector, highlighting its ripple effects on employee well-being and proactive behavior. It sheds light on the importance of fostering respectful interactions in public and private spaces, emphasizing mutual respect between customers and service providers. Recognizing the adverse impacts of mistreatment can prompt societal discourse on workplace ethics and encourage organizations to advocate for their employees' rights. Ultimately, nurturing a culture that condemns customer mistreatment can lead to healthier work environments, benefitting both employees and society at large.Originality/valueThis study presents a novel approach by investigating the impact of customer mistreatment on employee proactive behavior, considering insomnia as a mediator, a perspective that has received limited attention in existing literature. Additionally, it introduces the concept of resilience as a moderator, offering fresh insights into how individual resilience levels can affect the relationship between customer mistreatment and proactive behavior. The research goes beyond traditional analyses of workplace dynamics to explore the broader implications of these interactions on personal well-being and sleep patterns. Through the application of a moderated-mediation framework, this study enhances understanding of complex organizational behavior dynamics, particularly in the service sector, and provides valuable implications for both theoretical understanding and practical application.