1. The shiftworker's inability to adapt to shiftwork schedules can lead to a loss of physical and psychological well being, and can produce negative safety and performance consequences. 2. The factors that contribute to the intolerance of working shiftwork schedules are complex and interrelated and include host variables (i.e., circadian rhythms) and environmental variables (i.e., type of shifts worked). 3. The most direct and consistent effect of working shiftwork is the impairment of the quality and quantity of sleep. 4. Interventions should include a program that monitors workers' tolerance to shiftwork and provides information and recommendations for employees to effectively manage a lifestyle that incorporates nighttime work schedules.