This study of anesthesiology residents demonstrated that fatigue negatively impairs psychomotor functioning and mood but not measures of clinical performance. Investigators examined, scored, and analyzed the observed behaviors of 12 residents in differing states of prior sleep. The findings support the notion that fatigue can lead to errors as a result of impaired cognitive abilities even if the more difficult to measure clinical performance outcomes were less affected. These findings are the first from a comprehensive simulation study addressing the effects of provider fatigue.