A couple of quotes from the 16th and 17th centuries that still hold true today. The first from Leonardo da Vinci’s Notebooks, dated 1508:
Irons rusts from disuse, stagnant water loses its purity and in cold weather becomes frozen; even so does inaction sap the vigor of the mind.
Which reminded me of Robert Burton’s advice for combating depression, given in his landmark 17th century book, The Anatomy of Melancholy:
Observe this short precept — Be not solitary; be not idle.