Round-the-clock operational requirements pose physiological challenges for human operators. Fatigue due to sleep loss and circadian disruption can reduce safety, performance quality, and alertness. The authors describe the physiological factors underlying fatigue and provide examples from NASA research in aviation settings that demonstrate how fatigue affects real-world operations. A comprehensive alertness management approach to address fatigue effectively includes education, alertness strategies, scheduling, policy, and healthy sleep components. There is a need for cultural change that will encourage attitudes, behaviors, and practices that will reduce fatigue-related risks and improve safety, performance, and alertness in 24/7 operational settings.