Seneca, On Tranquillity of Mind 17.5
Seneca uses a farming analogy to argue that we must not overwork.
danda est animis remissio: meliores acrioresque requieti surgent. ut fertilibus agris non est imperandum (cito enim illos exhauriet numquam intermissa fecunditas), ita animorum impetus assiduus labor franget; vires recipient paulum resoluti et remissi. nascitur ex assiduitate laborum animorum hebetatio quaedam et languor.
We must give our minds a break: they will rise up better and sharper after being rested. Just as fertile fields must not be forced to produce (for fruitfulness which is never allowed a break will quickly exhaust them), so unremitting work will diminish the vigour of our minds; they will regain their strength little by little if they have been unyoked and allowed to relax. From unbroken labours is born a certain dullness and languor in our minds.