The effect of shift structure on worker performance and productivity is of increasing interest to firms and regulatory bodies. Using approximately 743,000 emergency medical incidents attended by 2,381 paramedics in Mississippi, we evaluate the extent that paramedics' performance toward the end of shifts is impacted by shift length. We find evidence that performance deteriorates toward the end of long shifts, and argue that fatigue is the mediating factor. Our calculations imply that such deterioration may result in a 0.76 percent increase in 30-day mortality. These findings have implications for workforce organization, calling attention to regulation designed to limit extended work hours. (JEL J22, J24, J28, J45, M12)