Objection 1. It would seem that the judgment of the intellect is not hindered by suspension of the sensitive powers. For the superior does not depend on the inferior. But the judgment of the intellect is higher than the senses. Therefore the judgment of the intellect is not hindered through suspension of the senses.
Objection 2. Further, to syllogize is an act of the intellect. But during sleep the senses are suspended, as is said in De Somn. et Vigil. i and yet it sometimes happens to us to syllogize while asleep. Therefore the judgment of the intellect is not hindered through suspension of the senses.
On the contrary, What a man does while asleep, against the moral law, is not imputed to him as a sin; as Augustine says (Gen. ad lit. xii, 15). But this would not be the case if man, while asleep, had free use of his reason and intellect. Therefore the judgment of the intellect is hindered by suspension of the senses.
I answer that, As we have said above (Article 7), our intellect's proper and proportionate object is the nature of a sensible thing. Now a perfect judgment concerning anything cannot be formed, unless all that pertains to that thing's nature be known; especially if that be ignored which is the term and end of judgment. Now the Philosopher says (De Coel. iii), that "as the end of a practical science is action, so the end of natural science is that which is perceived principally through the senses"; for the smith does not seek knowledge of a knife except for the purpose of action, in order that he may produce a certain individual knife; and in like manner the natural philosopher does not seek to know the nature of a stone and of a horse, save for the purpose of knowing the essential properties of those things which he perceives with his senses. Now it is clear that a smith cannot judge perfectly of a knife unless he knows the action of the knife: and in like manner the natural philosopher cannot judge perfectly of natural things, unless he knows sensible things. But in the present state of life whatever we understand, we know by comparison to natural sensible things. Consequently it is not possible for our intellect to form a perfect judgment, while the senses are suspended, through which sensible things are known to us.
Reply to Objection 1. Although the intellect is superior to the senses, nevertheless in a manner it receives from the senses, and its first and principal objects are founded in sensible things. And therefore suspension of the senses necessarily involves a hindrance to the judgment of the intellect.
Reply to Objection 2. The senses are suspended in the sleeper through certain evaporations and the escape of certain exhalations, as we read in De Somn. et Vigil. iii. And, therefore, according to the amount of such evaporation, the senses are more or less suspended. For when the amount is considerable, not only are the senses suspended, but also the imagination, so that there are no phantasms; thus does it happen, especially when a man falls asleep after eating and drinking copiously. If, however, the evaporation be somewhat less, phantasms appear, but distorted and without sequence; thus it happens in a case of fever. And if the evaporation be still more attenuated, the phantasms will have a certain sequence: thus especially does it happen towards the end of sleep in sober men and those who are gifted with a strong imagination. If the evaporation be very slight, not only does the imagination retain its freedom, but also the common sense is partly freed; so that sometimes while asleep a man may judge that what he sees is a dream, discerning, as it were, between things, and their images. Nevertheless, the common sense remains partly suspended; and therefore, although it discriminates some images from the reality, yet is it always deceived in some particular. Therefore, while man is asleep, according as sense and imagination are free, so is the judgment of his intellect unfettered, though not entirely. Consequently, if a man syllogizes while asleep, when he wakes up he invariably recognizes a flaw in some respect.
Source: Summa Theologica (New Advent)