For decades, much was made of the fact that American workers were the most productive of the Western world. Business articles derided the 35-hour work week of the French and Germans.
However, about 10 years ago, there was a study that showed that the French and German workers were much more productive per hour.
This supports my long-held belief that a typical worker averages 6-7 hours of productive work per day. Give them a short-term project and they will work harder and faster, but be less productive afterwards. Tell them that they are going to work overtime, and they will not work as hard in the regular day. Presented with too much work (for them), many will actually do less.
Realizing this is one of the things that has made my job (university teaching) better. I could do my work in less time, so that I had time for myself and my family, rather than twiddling my thumbs at my desk for 8 hours.
In short, people need time to goof off and socialize, and it makes them work better.