Further, rest at proper times is necessary for the framework of the body, that the nutriment may be diffused over the whole body through the passages which it contains, without any strain to hinder its progress. For just as certain misty vapours are drawn up from the recesses of the earth when it is soaked with rain, whenever the sun heats it with rays of any considerable warmth, so a similar result happens in the earth that is in us, when the nutriment within is heated up by natural warmth; and the vapours, being naturally of upward tendency and airy nature, and aspiring to that which is above them, come to be in the region of the head like smoke penetrating the joints of a wall: then they are dispersed thence by exhalation to the passages of the organs of sense, and by them the senses are of course rendered inactive, giving way to the transit of these vapours. For the eyes are pressed upon by the eyelids when some leaden instrument , as it were (I mean such a weight as that I have spoken of), lets down the eyelid upon the eyes; and the hearing, being dulled by these same vapours, as though a door were placed upon the acoustic organs, rests from its natural operation: and such a condition is sleep, when the sense is at rest in the body, and altogether ceases from the operation of its natural motion, so that the digestive processes of nutriment may have free course for transmission by the vapours through each of the passages.

Ἀναγκαία δὲ τῇ συστάσει τοῦ σώματος ἡ εὔκαιρος ἄνεσις͵ ὡς ἂν ἀκωλύτως ἐφ' ἅπαν τὸ σῶμα διὰ τῶν ἐν αὐτῷ πόρων ἡ τροφὴ διαχέοιτο͵ μηδενὸς τόνου τῇ διόδῳ παρεμποδίζοντος. Καθάπερ γὰρ ἐκ τῆς διαβρόχου γῆς͵ ὅταν ἐπιλάμψῃ θερμοτέραις ἀκτῖσιν ὁ ἥλιος͵ ἀτμοί τινες ὀμιχλώδεις ἀπὸ τοῦ βάθους ἀνέλκονται, ὅμοιόν τι γίνεται καὶ ἐν τῇ καθ' ἡμᾶς γῇ͵ τῆς τροφῆς ἔσωθεν ὑπὸ τῆς φυσικῆς θερμότητος ἀναζεούσης. Ἀνωφερεῖς δὲ ὄντες οἱ ἀτμοὶ κατὰ φύσιν καὶ ἀερώδεις καὶ πρὸς τὸ ὑπερκείμενον ἀναπνέοντες ἐν τοῖς κατὰ τὴν κεφαλὴν γίνονται χωρίοις͵ οἷόν τις καπνὸς εἰς ἁρμονίαν τοίχου διαδυόμενος· εἶτα ἐντεῦθεν ἐπὶ τοὺς τῶν αἰσθητηρίων πόρους ἐξατμιζόμενοι διαφοροῦνται͵ δι' ὧν ἀργεῖ κατ' ἀνάγκην ἡ αἴσθησις͵ τῇ παρόδῳ τῶν ἀτμῶν ἐκείνων ὑπεξιοῦσα. Αἱ μὲν γὰρ ὄψεις τοῖς βλεφάροις ἐπιλαμβάνονται͵ οἷόν τινος μηχανῆς μολυβδίνης͵ τοῦ τοιούτου λέγω βάρους͵ τοῖς ὀφθαλμοῖς ἐπιχαλώσης τὸ βλέφαρον. Παχυνθεῖσα δὲ τοῖς αὐτοῖς τούτοις ἀτμοῖς ἡ ἀκοή͵ καθάπερ θύρας τινὸς τοῖς ἀκουστικοῖς μορίοις ἐπιτεθείσης͵ ἡσυχίαν ἀπὸ τῆς κατὰ φύσιν ἐνεργείας ἄγει· καὶ τὸ τοιοῦτον πάθος ὕπνος ἐστίν͵ ἀτρεμούσης ἐν τῷ σώματι τῆς αἰσθήσεως [168] καὶ παντάπασιν ἐκ τῆς κατὰ φύσιν κινήσεως ἀπρακτούσης͵ ὡς ἂν εὐπόρευτοι γένωνται τῆς τροφῆς αἱ ἀναδόσεις͵ δι' ἑκάστου τῶν πόρων τοῖς ἀτμοῖς συνδιεξιούσης.