Actually, in the US, it was the labor unions. They were demanding 8 hour work days as a part of their labor contracts in the.... mid 1800s. In 1868 Congress passed a law so the federal work force worked an 8 hour day. On May 1st, 1886, 80,000 people marched down Michigan Ave in Chicago for an 8 hour work day.
But you can stick with your capitalist fantasy about how "Henry Ford discovered" the 8 hour work day and gifted it to his workers so they could be more productive.