The results reviewed clearly demonstrate that shift work is associated with increased subjective, behavioral, and physiological sleepiness. Apparently, the effects are due to combined circadian and homeostatic (sleep loss) influences. Sleepiness is particularly pronounced during the night shift, and may terminate in actual incidents of falling asleep at work. In some occupations this clearly constitutes a hazard that may endanger human lives and have large economic consequences. These risks clearly involve a larger number of people and should be of great significance to society.