Just curious, as human race, what makes us think 40-hour(or any numbers of hours) is the norm that everyone should stick to? if we switch to 4-weekday in the f

Just curious, as human race, what makes us think 40-hour(or any numbers of hours) is the norm that everyone should stick to? if we switch to 4-weekday in the future, we will then consider 4*8=32 hour per week is typical, and work 40+ hours a week will burn you out? where and how to draw the line? I agree that there is no point to brag about your 60+ hour work style, but, what really decides the 'normal' work hours each week?