With abundant health-related information, the modern workforce is advised to engage in health-promoting behaviors such as good sleep, physical activities, and a healthy diet to stay productive at work. However, no study has provided a theoretical framework or empirical evidence on the association between employees' unhealthy eating behavior and the quality of their performance. Drawing from the stress and coping literature, the current study proposes a moderated mediation model to investigate the day-specific roles of (un)healthy lifestyle in regard to personal well-being and performance at work. We used daily diary data collected from 97 full-time employees and employed an experience sampling method (ESM) to examine this within-person phenomenon for 2 weeks. Our multilevel path analysis reveals that employees' unhealthy eating behavior in the evening led to emotional strain (e.g., guilt) as well as physical strain (e.g., stomachache, diarrhea) on the next morning; the emotional and physical strains experienced in the morning served as key mediators resulting in decreased quality of performance (i.e., less helping and more withdrawal behaviors) in the afternoon. Furthermore, emotional stability was found to moderate the relationship between unhealthy eating behavior and morning strains, such that employees with higher emotional stability tended to experience less negative emotions and fewer physical symptoms. The theoretical and practical implications of these findings are discussed, along with suggestions for future studies on health-related behaviors. (PsycInfo Database Record (c) 2022 APA, all rights reserved).