In the strategic HR literature, current empirical results on the relationship between HR practices and employee wellbeing are mixed and contradictory. Based on the job resources and demands model and the fine-tuned challenge-hindrance demands framework, we propose that an important reason lies in the lack of attention paid to the different characteristics of HR practices. HR practices can serve as either job resources or challenge demands to employees, thereby having differential effects on the psychological, physical, and social dimensions of wellbeing. We integrate a measure of challenge demand (including time pressure and workload) as a mediator to further reveal how these different categories of HR practices influence employee wellbeing. Using structural equation modeling in a dataset of 4823 individual workers from a National Workplace Survey of Employees conducted in Ireland, we find that job resource HR practices are positively associated with all three dimensions of wellbeing both directly and indirectly, while challenge demand HR practices are positively associated with psychological wellbeing but negatively associated with physical wellbeing and social wellbeing primarily through the mediating effect of time pressure and workload. These findings point to important variable relationships, reinforcing the need to untangle the HRM employee wellbeing relationship beyond aggregated and uniform HRM-wellbeing assertions.