The eight-hour workday is a tired and old concept. It's time to put aside this tired, arcane way of thinking about work. In order to understand how to fix it, we have to understand why we have the eight-hour workday to begin with.
In the late 1800s, full-time manufacturing employees would work around 100 hours per week. Death and injury were commonplace. It wasn't uncommon for children to put in far more hours than we think reasonable today in the factories, fields, mills, and mines.