Shift work causes impaired sleep, markedly increased sleepiness/fatigue and increased accident risk. The night shift constitutes the main problem but also early morning work causes similar problems. About 10% of the shift working population has severe problems and the diagnosis “Shift Work Disorder” may be applied. Particular characteristics, such as short rest between shifts, and long shifts, may exacerbate the problem. Countermeasures include napping, light treatment, extended rest breaks.