OBJECTIVE: The purpose of this article is to describe the impact of shift work on sleep, as recently acknowledged in official nosologies of sleep disorders, and to discuss whether sleep altered by shift work actually constitutes a disorder. METHOD: The authors review subjective responses to recent survey questions about sleep and polygraphic measurements of sleep in shift workers and describe sleep clinic experiences with complaints related to shift work. FINDINGS: Shift work entails wide variation in work schedules, sleep quality, and worker tolerance and a high prevalence of night-shift sleepiness. It probably affects rates of drug use, health status, and family organization. Clinical presentations were rare, highly varied, and empirically treated. The United States, unlike other countries, has no legal restrictions on shift work. CONCLUSIONS: As a clinical phenomenon, sleep altered by shift work is common and varied, probably expresses nonphysiological sleep-wake scheduling, and is little treated. Further study of its health effects and consideration of whether it is a "disorder" or a "problem" seem warranted.Abstract Teaser