The concept of a 40-hour work week was invented by Henry Ford

Henry Ford introduced the 40-hour, five-day work week at Ford Motor Company in 1926 — not as a labour rights gesture but as a business calculation. He had observed that working hours beyond 40 per week produced diminishing output and rising error rates. He also reasoned that workers who had two days off and decent wages would spend money on consumer goods — including cars. The 40-hour standard was enshrined in US law by the Fair Labor Standards Act in 1938. The irony is that Ford's calculation remains unchallenged in most workplaces despite almost a century of additional evidence that knowledge workers may be most productive working far fewer hours.

Source: Alterman, A Short History of the 40-Hour Work Week, CounterPunch (2009)