Most people take it for granted, that a normal work week of full pay without overtime is 40 hours, and thereby conclude that it always has been 40 hours and so forth, but that is a long, long way from the truth. The truth of the matter is, as society gravitated from being an agricultural-based economy into the industrial revolution and service industry, the rules of the hours necessitated for those laborers working for management were not established at eight hours a day, but were considerably longer, often necessitating 12 hours per day, six days a week. Not only were the hours long, but the pay for those hours, often was sufficient only for the basic necessities of life, and nothing much more.
Not too surprisingly, those working the long hours, recognized that although each individual was in of himself, essentially powerless to effect change for their labor, recognized though that groups of individuals united in the purpose of achieving both better pay as well as shorter hours, could achieve change. This meant that even in the late 18th century, in cities such as Philadelphia, then the second biggest city in America, labor strikes by carpenters, for instance, occurred, demanding that the standard day should be reduced from 12 hours a day to just 10 hours a day. This agitation by labor for reduced working hours was to continue for decades, in which, through starts and stops, through strikes and violence, through voluntary cooperation and court order, the tide slowly began to turn so as to provide more fairness to the common laborer, who seemed entitled in a free nation to appropriate leisure as well as rest time.
In the aftermath, of the devastating civil war, President Grant issued a proclamation in 1868, declaring that for federal workers, eight hour workdays would become the norm, but despite this proclamation, the workaround for this new charge, was to reduce wages to reflect the less hours worked, and the courts while recognizing the validity of the new law, essentially declare it not "obligatory", since wages and hours worked must be "…determined by the inexorable laws of business." Nevertheless, this proclamation gave new impetus for the labor movement at large to work harder at achieving their goals of “8 hours for work, 8 hours for rest, and 8 hours for what we will.”
In 1914, Henry Ford, Founder of the Ford Motor Company, determined that not only was 48 hours a week, too long for workers to labor diligently, but that reducing the work week to 40 hours a week along with actually increasing pay would be good for both the company as well as the worker. Ford reasoned that if his laborers made more money that they would thereby spend that money buying the very products that they created, increasing profits, as well as being able to enjoy thereby the fruits of their labor, something that historically had been seen as a "class privilege". Ford's foresight was instrumental in the continuous push for universal eight hour work days.
In 1938, Roosevelt signed into law the Fair Labor Standards Act, which established both a federally mandated minimum wage as well as setting 44 hours as the standard work week, to which, this was phased out to both a higher minimum wage as well as lowering the standard work week to 40 hours over a period of seven years. The Fair Labor Standards Act, though amended, is still applicable today, to which, both eight hour days as well as 40-hour work weeks are the baseline standard for American workers.