This paper reviews the effects of shift work and finds strong, acute effects on sleep and alertness in relation to night and morning work. The effects seem, however, to linger and also affect days off. The level of the disturbances is similar to that seen in clinical insomnia and may be responsible for considerable human and economical costs due to fatigue related accidents and reduced productivity. The mechanism behind the disturbances is the sleep interfering properties of the circadian system during day sleep and the corresponding sleep promoting properties during night work. Various strategies may be used to counteract the effects of shift work, such as napping, sufficient recovery time between shifts, clockwise rotation, etc. Still it does not seem possible to more eliminate the effects-only to reduce them.