The eight-hour workday was developed during the Industrial Revolution so that workers doing manual labor in a factory would not have to work as many hours. Prior to that, in the late 18th century, factory workers worked 10-16 hours days to keep factories running 24/7. A campaign was started to have people work no more than eight hours in a day so that they would have eight hours of work, eight hours of recreation, and eight hours of rest. The campaign was successful.
The goal was humane, but it doesn’t take into account our modern situation in which employees who work steadily for hours on end, sometimes for 10 to 12 hours in a day, are assumed to be more productive than employees who take breaks. According to the Bureau of Labor Statistics, the average American actually works 8.8 hours in a day. This is more time than the average worker spends doing anything else.
However, one study shows that workers who take brief breaks are more productive than those who keep working continuously for more hours, regardless of the number of hours they were working overall. Ideally, according to that study, there is an hour of uninterrupted work and then a break of about 15-20 minutes.