Shift work disrupts circadian and homeostatic sleep regulation and is probably the most common cause of sleepiness in society. This chapter talks about the shift work disorder, and presents survey studies of sleepiness, physiological sleepiness and momentary subjective sleepiness. EEG measures give some indication of the pattern of sleepiness in relation to shift work, that is, incidents of sleep intrusions are frequently seen during the second half of a night shift. If sleepiness is as pronounced as suggested in connection with night work, there should be a link with performance errors and accidents. Shift work that includes night shifts causes high levels of sleepiness that may constitute a major safety risk, particularly in transport work, but also in health care and industry. The chapter discusses the individual differences, the diagnosis shift work disorder, and possible ways of counteracting or treating intolerance to shift work.