Now just as weariness of the body is dispelled by resting the body, so weariness of the soul must needs be remedied by resting the soul: and the soul's rest is pleasure, as stated above (I-II, 25, 2; I-II, 31, 1, ad 2). Consequently, the remedy for weariness of soul must needs consist in the application of some pleasure, by slackening the tension of the reason's study. Thus in the Conferences of the Fathers xxiv, 21, it is related of Blessed John the Evangelist, that when some people were scandalized on finding him playing together with his disciples, he is said to have told one of them who carried a bow to shoot an arrow. And when the latter had done this several times, he asked him whether he could do it indefinitely, and the man answered that if he continued doing it, the bow would break. Whence the Blessed John drew the inference that in like manner man's mind would break if its tension were never relaxed.
Thomas Aquinas, Summa Theologiae 2-2.168.2. Quies animae est delectatio: we could also translate it as 'The quiet of the soul is delight', and 'delight' perhaps gets us closer than 'pleasure' to what Aquinas means here.