Shift work was made possible by the invention of the light bulb, and the conveyor belt. Shift work enables a factor to get as much as four times the work output. Shift work is very difficult for the workers whose sleep is disrupted. When workers sleep is disrupted by ever-changing schedules, their efficiency is actually reduced. The risk of accidents rises sharply.

In Europe an experiment to try and help workers with the problems of adjusting to new schedules. In Europe the shift that workers are on can change every two days. The idea was that by not giving their body a chance to get used to any particularly schedule, the problems of adjusting to new schedules could be minimized. Unfortunately, our bodily clock does not allow such a plan to work. We are programmed with a very deep need to sleep between 1am and 4am, and again a lesser but significant need 12 hours later in the afternoon, between 1pm and 4pm. 

Our bodies can adjust to a new schedule. But researchers have found that we have several different biological clocks, and each of these clocks tends to adjust at a different rate. It can take as much as a week to get used to a new schedule and for all the different bodily clocks to get back in sequence. Some of the more important clocks adjust at the rate of about 1 hour per day. Because we seem to have a biological clock whose main rhythm is set at 25 hours, adjusting to a later schedule is easier than an earlier schedule. However, shift workers schedules are usually changed in a counter-clockwise fashion!  This is probably the most disruptive direction. Experiments of changing schedules in a clockwise direction show that worker satisfaction and performance is increased.

Another way to improve the functioning of shift workers is to let them choose a certain shift, and let them stay with that shift. Absentism, depression, accidents, have all been shown to decrease when such a plan is implemented. Worker satisfaction improves. But, the workers should make sure that they maintain the same sleeping schedule on their days off, and supplement and needed changes with naps. If they don't do these things, they will loose much of the benefit of a fixed shift.