In this study, we examined whether the expression of voice, as a prosocial yet psychologically costly behavior, during the day affects employee sleep quality at night and whether sleep quality affects the expression of voice on the next workday. Having surveyed 113 full-time employees twice a day over the course of two weeks, we found that the same-day effect of voice on sleep and the effect of sleep on the next day voice depend on the type of voice. Specifically, we found that employees who express promotive voice experience higher levels of positive affect at the end of the workday, more effectively detach from work at home, and as a result are less likely to suffer from insomnia at night. We also found that employees who express prohibitive voice tend to experience higher levels of negative affect at the end of the workday, are unable to effectively detach from work at home, and consequently are more likely to experience insomnia at night. Our study further demonstrates that, while there is no association between insomnia and next-day prohibitive voice, sleep-deprived employees are less likely to engage in promotive voice on the next day because of being psychologically depleted.