Athenaeus of Attalia was one of the first doctors we know to work out a unified medical theory that covered both mental and physical health and habits. Before Athenaeus it was more often the business of philosophers to talk about good and bad mental habits, and of doctors to talk about physical habits like eating and exercise. Athenaeus, however, consciously brings philosophy and medicine together into one discipline, speaking interchangeably about habits of either mind or body. He calls the effects of these behaviours, "habituation", and our habituation in turn determines what things are good for us and what things are bad.
Athenaeus thinks of habituation as a kind of "state" or "innate disposition", but he also calls it (perhaps following Aristotle) "second nature", probably to emphasize that its effects on both body and mind can be quite permanent. There's a long tradition in Greco-Roman thought that treats art as something subordinate to nature; however, physicians (and farmers) were well aware that art can get the upper hand. Here, Athenaeus explains how. His thoughts were preserved by Oribasius.
"From the works of Athenaeus. On Habituation.
"Habituation is a state of the soul or body established over time with respect to benefit or harm when we are healthy or sick. For habit over time establishes something through itself in the soul and in the body, and this sometimes makes something beneficial, sometimes [something] harmful. Not only is it strong in times of health, but it often extends even into times of illness. And a habit that lasts for a long time is like an acquired nature. For this reason, if any self-mover undergoes a change [?], it is dangerous and introduces disease. Of these [changes], changes of place make a big difference: for change from healthy [places] to more diseased ones produces a greater and more serious alteration, while the [change] from diseased [places] to healthy ones [produces] a smaller [alteration] in both magnitude and duration. Indeed, every change, especially a sudden one, and most especially one which is not customary and unusual, alters bodies for the worse for the reason we gave. Habituation is so powerful that those who have been seized by it cannot exist separately [from it], being held in its bonds. An athlete, for example, has difficulty recovering: if he wishes to return to his original way of life by suddenly abolishing his acquired disposition, just like a second nature, then he will quickly be destroyed, because a way of life continued for a long time is (so to speak) a powerful form of being accustomed. Thus, a sudden departure from habit makes a great difference. For this reason it drives out of their proper place of rest those who do not abolish the earlier habituation gradually and by means of another, different habituation."
Ἐκ τῶν Ἀθηναίου. Περὶ συνηθείας.
Συνήθειά ἐστιν ἕξις ψυχῆς ἢ σώματος ἐν χρόνῳ κατεσκευασμένη πρὸς ὠφέλειάν τε καὶ βλάβην ὑγιαινόντων τε καὶ νοσούντων· τὸ γὰρ ἔθος ἐν χρόνῳ κατασκευάζει τι δι’ ἑαυτοῦ περὶ τὴν ψυχὴν καὶ τὸ σῶμα, καὶ τοῦτό ποτε μὲν ἐπ’ ὠφέλειαν ποιεῖ τινα, ποτὲ δὲ καὶ ἐπὶ βλάβην. καὶ οὐ μόνον ἐφ’ ὑγιαινόντων ἰσχύει, διατείνει δὲ πολλάκις καὶ πρὸς τοὺς νοσοῦντας. τὸ δὲ πολυχρόνιον ἔθος οἷον φύσις ἐστὶν ἐπίκτητος· διὸ πᾶν τὸ κινοῦν ἑαυτὸ μεταβάλλει, ἐπισφαλὲς καὶ προσαγωγὸν εἰς νόσον. τούτων δὲ διαφέρουσιν αἱ μεταβολαὶ τῶν τόπων· ἡ μὲν γὰρ ἐξ ὑγιεινῶν εἰς νοσερώτερα μεταβολὴ μείζονα ποιεῖ τὴν ἀλλοίωσιν καὶ χαλεπωτέραν, ἡ δ’ ἐκ νοσερῶν εἰς ὑγιεινὰ ἐλάσσονα καὶ τῷ μεγέθει καὶ τῷ χρόνῳ. πᾶσα μέντοι μεταβολή, καὶ μάλιστα αἰφνίδιος, καὶ ὡς ἔνι μάλιστα ἀσυνήθης καὶ ξένη, ἐπὶ τὸ χεῖρον μετακινεῖ τὰ σώματα δι’ ἣν ἀπεδώκαμεν αἰτίαν. οὕτως δ’ ἰσχυρὸν ἡ συνήθεια, ὥστε τοὺς ληφθέντας ὑπ’ αὐτῆς μηδὲ χωρισθῆναι δύνασθαι δεσμῷ κατεχομένους· δυσανάληπτος γὰρ ἀθλητής, εἰ θέλει πρὸς τὸν πρῶτον ἐπανελθεῖν βίον αἰφνιδίως τὴν ἐπίκτητον διάθεσιν ὥσπερ δευτέραν τινὰ φύσιν καταλύων, συντόμως ἀναλυθήσεται· οἷον γὰρ τρόπος ὁ πολυχρόνιος ἐθισμὸς ἰσχυρός. οὕτως ὁ αἰφνίδιος ἐξεθισμὸς μεγάλας ἔχει τὰς διαφοράς· διόπερ ἐξίστησι τῆς ἰδίας καταπαύσεως τοὺς μὴ ἐκ προσαγωγῆς καὶ δι’ ἑτέρας πάλιν συνηθείας τὴν προτέραν καταλύοντας συνήθειαν.
Athenaeus ap. Oribasius, libri incerti 17 (CMG VI 2,2 106,8-29 Raeder)
 cf. Sextus Empiricus, PH 1.146.3-4: ἔθος δὲ ἢ συνήθεια (οὐ διαφέρει γάρ) πολλῶν ἀνθρώπων κοινὴ πράγματός τινος παραδοχή. Sextus treats "habituation" and "habit" as synonyms, whereas Athenaeus considers habituation to be the result of habits over a long period of time.
 Must be ἐπ ὠφελειᾳ / ἐπὶ βλάβῃ