This paper studies the link between working hours and productivity using daily informationon working hours and performance of a sample of call centre agents. We exploit variationin the number of hours worked by the same employee across days and weeks due tocentral scheduling, enabling us to estimate the effect of working hours on productivity. Wefind that as the number of hours worked increases, the average handling time for a callincreases, meaning that agents become less productive. This result suggests that fatiguecan play an important role, even in jobs with mostly part-time workers.