Aeneas Silvius Piccolomini, On the Education of Boys:
One must not always press on in literature and serious affairs, nor should young students be tasked with immense labors, under which they lapse under exhaustion and find themselves oppressed by the burden of their grievances, such that they understand their studies with less ready facility. Just so, plants may be nourished by seasonable applications of water, but drowned by an excess of the very same. We should remember that our life is divided into two parts: study and relaxation. Just as there is waking and sleep, war and peace, summer and winter, work days and holidays, so is leisure the condiment of labor. And so, one should neither take on too much work, or indulge in excessive leisure. For as Plato says, the enemies of learning are work and sleep.
Non est semper litteris seriosisque rebus incumbendum, nec immensi labores sunt pueris adiiciendi, sub quibus defessi corruant et alioquin pondere molestiarum oppressi, doctrinam minus mansuete percipiant. Plantae namque cum modicis alantur aquis, quae multis suffocantur. Nosse oportet vitam nostram in duas partes esse divisam: in studium ac remissionem. Sic vigiliae somnus, pax bellum, aestas hiems, operosi festique dies, laboris condimentum est otium. Itaque nec laborem sumere nimium nec otio nimis indulgere oportet. Hostes enim disciplinarum, ut a Platone dictum est, labores atque somni.