The 40 hour work week exists because Henry Ford found through studies that productivity increased when employees worked no more than 8 hours/day, 40 hours/week.

The 40 hour work week exists because Henry Ford found through studies that productivity increased when employees worked no more than 8 hours/day, 40 hours/week. That was assembly line work in the early 20th century, and since then, I'm not aware of any other studies done about worker productivity and the amount of time worked.