Reducing duty hours for physicians in training should, in theory, improve patient safety by reducing physician fatigue. Indeed, prior research documents a link between increased fatigue and self-reported errors and percutaneous injuries among residents. But do increased work hours directly lead to fatigue? This survey of interns in cognitive specialties (including internal medicine, pediatrics, and psychiatry) at an academic medical center found that the major determinants of fatigue were increased stress level and poorer quality of sleep—not the absolute number of hours worked. Working more than 80 hours per week (the maximum, according to current regulations) was not associated with increased stress or fatigue. This study adds to a growing body of research that questions the relationship between work hours and housestaff fatigue. A recent editorial called for evidence-based duty hours regulations, taking into account not only hours worked per week but overall housestaff workload and other system factors contributing to stress and fatigue.