How does sleep affect employee effectiveness and what can employees do to remain effective on days with a lack of sleep? Drawing on the conservation of resources theory our research expands on the cognitive (regulatory resources), affective (positive affect), and motivational (subjective vitality) mechanisms that link sleep and employee effectiveness. Furthermore, considering the crucial role of individual beliefs in the spillover of sleep to work, we examine the moderating role of implicit theories about willpower – a mindset about the resource-draining nature of self-regulation – in the relation between sleep duration and employee effectiveness through regulatory resources availability. Two daily diary studies with a combined sample of Ntotal=214 employees (Ntotal=1317 workdays) demonstrate the predominant role of cognitive and affective resources in linking sleep at home to engagement, in-, and extra-role performance at work. Moreover, the spillover of sleep to employee effectiveness via cognitive resources is stronger for individuals holding a limited as compared to a non-limited resource theory. This research not only expands our theoretical understanding of the psychological mechanisms that link sleep to employee effectiveness but also offers practical implications by highlighting the protective role of holding a non-limited resource theory on days with a lack of sleep.