||In this research, we draw from the stereotyping literature to suggest that supervisor ratings of job performance are affected by employees’ start times—the time of day they first arrive at work. Even when accounting for total work hours, objective job performance, and employees’ self-ratings of conscientiousness, we find that a later start time leads supervisors to perceive employees as less conscientious. These perceptions in turn cause supervisors to rate employees as lower performers. In addition, we show that supervisor chronotype acts as a boundary condition of the mediated model. Supervisors who prefer eveningness (i.e., owls) are less likely to hold negative stereotypes of employees with late start times than supervisors who prefer morningness (i.e., larks). Taken together, our results suggest that supervisor ratings of job performance are susceptible to stereotypic beliefs based on employees’ start times.