I couldn't find the individual who came up with the idea but I did find some info about how it became the standard:
"Demands for an eight-hour working day became increasingly widespread among American laborers in the 1880s. In 1866, the National Labor Union failed to convince Congress to legalize the eight-hour working day...It wasn't until 1938 when Congress proposed the Fair Labor Standards Act, establishing a minimum wage, initially $0.25 an hour, along with a maximum workweek of 44 hours; these were to become $0.40 an hour and 40 hours after seven years.
The Fair Labor Standards Act has been amended repeatedly in subsequent decades, with changes expanding the classes of workers covered; raising the minimum wage; redefining regular-time work and raising overtime payments so as to encourage the hiring of new workers, as opposed to the loading of extra work on the lowest-paid; and equalizing pay scales for men and women.
The Work Hours Act of 1962 provided time-and-a-half pay for work over an 8-hour day or a 40-hour week."

100 year ago and more, most people worked from dawn until dusk, including children, with only short breaks for meals. In the summer, that meant the workday could last from 6 a.m. or earlier until 9 p.m. or later. 12 hour workdays were easily the norm. There was even a big movement early on to limit it to a 10-hour workday!

After many bloody riots and revolts, laws were finally passed in both England and the U.S. (as well as eventually other European countries) limiting the workday to 8 hours.

There was talk in the 70's about how all the new time-saving conveniences were going to result in a culture with more leisure time than ever before. I remember seeing one article when I inherited a bunch of old magazines that stated that by the year 2000 the average workday would be 4 hours! The writer didn't realize that rather than let employees just go home, employers wanted the higher productivity and profits.