The use of extended work shifts and overtime has escalated as hospitals cope with a shortage of registered nurses (RNs). Little is known, however, about the prevalence of these extended work periods and their effects on patient safety. Logbooks completed by 393 hospital staff nurses revealed that participants usually worked longer than scheduled and that approximately 40 percent of the 5,317 work shifts they logged exceeded twelve hours. The risks of making an error were significantly increased when work shifts were longer than twelve hours, when nurses worked overtime, or when they worked more than forty hours per week.