42 If we turn now to things external and material, the first thing of all to secure is to save unfortunate working people from the cruelty of men of greed, who use human beings as mere instruments for money-making. It is neither just nor human so to grind men down with excessive labor as to stupefy their minds and wear out their bodies. Man's powers, like his general nature, are limited, and beyond these limits he cannot go. His strength is developed and increased by use and exercise, but only on condition of due intermission and proper rest. Daily labor, therefore, should be so regulated as not to be protracted over longer hours than strength admits. How many and how long the intervals of rest should be must depend on the nature of the work, on circumstances of time and place, and on the health and strength of the workman. Those who work in mines and quarries, and extract coal, stone and metals from the bowels of the earth, should have shorter hours in proportion as their labor is more severe and trying to health. Then, again, the season of the year should be taken into account; for not unfrequently a kind of labor is easy at one time which at another is intolerable or exceedingly difficult. Finally, work which is quite suitable for a strong man cannot rightly be required from a woman or a child. And, in regard to children, great care should be taken not to place them in workshops and factories until their bodies and minds are sufficiently developed. For, just as very rough weather destroys the buds of spring, so does too early an experience of life's hard toil blight the young promise of a child's faculties, and render any true education impossible. Women, again, are not suited for certain occupations; a woman is by nature fitted for home-work, and it is that which is best adapted at once to preserve her modesty and to promote the good bringing up of children and the well-being of the family. As a general principle it may be laid down that a workman ought to have leisure and rest proportionate to the wear and tear of his strength, for waste of strength must be repaired by cessation from hard work. In all agreements between masters and work people there is always the condition expressed or understood that there should be allowed proper rest for soul and body. To agree in any other sense would be against what is right and just; for it can never be just or right to require on the one side, or to promise on the other, the giving up of those duties which a man owes to his God and to himself.
Source: Rerum Novarum (Vatican.va)