Employee overwork and fatigue are a concern of managers in many organizations, as they may increase health and safety risks and decrease productivity. The problem is especially severe in competitive environments, where compensation and promotions are awarded, explicitly or implicitly, on the basis of relative performance. We propose a theory for, and study experimentally, the phenomenon of fatigue in a dynamic competitive environment. We find that subjects react strongly to changes in the environment related to fatigue and follow the comparative statics of equilibrium predictions. At the same time, within a given environment, subjects behave as if they are unaware of the deteriorating effect of fatigue on their competitiveness.