Organizational scholars have examined a number of antecedents of insomnia in a search for ways to prevent insomnia and its negative implications for the workplace. However, most studies have focused on the antecedents that are beyond employee control. Therefore, our collective understanding of how employees can modify their workplace behaviors to reduce the symptoms of insomnia and prevent its adverse consequences has remained limited. In this study, we examined whether the expression of voice, as a prosocial yet psychologically costly behavior that is under employee control, affects employee sleep quality, and whether sleep quality affects the expression of voice on the next workday. Having surveyed 113 full-time employees twice a day for 10 workdays, we found that employees who express promotive voice at work experience higher positive affect at the end of the workday, more effectively detach from work in the evening, and are less likely to suffer from insomnia at night. We also found that employees who express prohibitive voice at work experience higher negative affect at the end of the workday, less effectively detach from work in the evening, and are more likely to experience insomnia at night. Our study further demonstrates that, while insomnia is not related to the expression of prohibitive voice on the next day, sleep-deprived employees are less likely to engage in promotive voice because of being psychologically depleted. The results of our study suggest that sleep problems might be mitigated if employees regulate their engagement in costly workplace behaviors, such as voice. (PsycInfo Database Record (c) 2023 APA, all rights reserved).