Why is 8 hours a working-day?

Answer 1

I'm not sure, but the maximum you can work is 24 hours. There are a number of people who work "odd" shifts that have stints of 10, 12 and even 24 hours. Firefighters, hospital workers and others have some long days. There are reasons for limiting the length of a work day, but there is a need for the rigorous schedules that include longer shifts. You could work 4 10-hour days driving a hauler in an open pit mine and get your 40-hour week in and a 3-day weekend. And there are other possibilities in the world of work.

Answer 2

Because for much of recent history, urban working hours were 9 am to 5 pm = 8 hours.