Objection 1. It would seem that sleep and baths do not assuage sorrow. For sorrow is in the soul: whereas sleep and baths regard the body. Therefore they do not conduce to the assuaging of sorrow.
Objection 2. Further, the same effect does not seem to ensue from contrary causes. But these, being bodily things, are incompatible with the contemplation of truth which is a cause of the assuaging of sorrow, as stated above (Article 4). Therefore sorrow is not mitigated by the like.
Objection 3. Further, sorrow and pain, in so far as they affect the body, denote a certain transmutation of the heart. But such remedies as these seem to pertain to the outward senses and limbs, rather than to the interior disposition of the heart. Therefore they do not assuage sorrow.
On the contrary, Augustine says (Confess. ix, 12): "I had heard that the bath had its name [Balneum, from the Greek balaneion]... from the fact of its driving sadness from the mind." And further on, he says: "I slept, and woke up again, and found my grief not a little assuaged": and quotes the words from the hymn of Ambrose [Cf. Sarum Breviary: First Sunday after the octave of the Epiphany, Hymn for first Vespers], in which it is said that "Sleep restores the tired limbs to labor, refreshes the weary mind, and banishes sorrow."
I answer that, As stated above (Question 37, Article 4), sorrow, by reason of its specific nature, is repugnant to the vital movement of the body; and consequently whatever restores the bodily nature to its due state of vital movement, is opposed to sorrow and assuages it. Moreover such remedies, from the very fact that they bring nature back to its normal state, are causes of pleasure; for this is precisely in what pleasure consists, as stated above (Question 31, Article 1). Therefore, since every pleasure assuages sorrow, sorrow is assuaged by such like bodily remedies.
Reply to Objection 1. The normal disposition of the body, so far as it is felt, is itself a cause of pleasure, and consequently assuages sorrow.
Reply to Objection 2. As stated above (Question 31, Article 8), one pleasure hinders another; and yet every pleasure assuages sorrow. Consequently it is not unreasonable that sorrow should be assuaged by causes which hinder one another.
Reply to Objection 3. Every good disposition of the body reacts somewhat on the heart, which is the beginning and end of bodily movements, as stated in De Causa Mot. Animal. xi.
Source: Summa Theologica (New Advent)