Pretty much nobody had 8 hour work days. One of the earliest known instances of someone demanding, and getting, an 8 hour workday was a English-born settler in New Zealand, who worked as a carpenter. In 1840, he negotiated an 8 hour working day for some building work, and he started arguing for it with other workers:
Prior to the 1890's, it took decades to even get down to the 12, and then 10 hour workday.
I don't remember specifically when there started being actual research into the productivity effects of shorter work days, but it definitively was not known at the outset - at the outset, it was driven by workers rights movements.
http://en.wikipedia.org/wiki/Samuel_Duncan_Parnell
Here's a PDF that gives some stats from 1890 until the 1930's: http://www.nber.org/chapters/c4124.pdf
Prior to the 1890's, it took decades to even get down to the 12, and then 10 hour workday.
I don't remember specifically when there started being actual research into the productivity effects of shorter work days, but it definitively was not known at the outset - at the outset, it was driven by workers rights movements.