This probably doesn't answer your question, but when I was in grad school 30 years ago, there was an unwritten assumption that limiting your hours was necessary

This probably doesn't answer your question, but when I was in grad school 30 years ago, there was an unwritten assumption that limiting your hours was necessary, so you could spend most of your time focusing on "getting done." Staying in school imposed a savage opportunity cost compared to finishing quicker and getting into a higher paying job. It also imposed a risk, since there were a lot of things that could happen, outside of your control, and knock you out of the program without a degree. I worked my extra hours as a musician, and actually made decent money compared to my graduate stipend. But it was limited to about 4 hours a week. And it was a source of recreation and "escape" that I needed.