Objection 1. It would seem that nocturnal pollution is a sin. For the same things are the matter of merit and demerit. Now a man may merit while he sleeps, as was the case with Solomon, who while asleep obtained the gift of wisdom from the Lord (1 Samuel 3:5). Therefore a man may demerit while asleep; and thus nocturnal pollution would seem to be a sin.
Objection 2. Further, whoever has the use of reason can sin. Now a man has the use of reason while asleep, since in our sleep we frequently discuss matters, choose this rather than that, consenting to one thing, or dissenting to another. Therefore one may sin while asleep, so that nocturnal pollution is not prevented by sleep from being a sin, seeing that it is a sin according to its genus.
Objection 3. Further, it is useless to reprove and instruct one who cannot act according to or against reason. Now man, while asleep, is instructed and reproved by God, according to Job 33:15-16, "By a dream in a vision by night, when deep sleep is wont to lay hold of men [Vulgate: 'When deep sleep falleth upon men.' St. Thomas is apparently quoting from memory, as the passage is given correctly above, 95, 6, Objection 1... Then He openeth the ears of men, and teaching instructeth them in what they are to learn." Therefore a man, while asleep, can act according to or against his reason, and this is to do good or sinful actions, and thus it seems that nocturnal pollution is a sin.
On the contrary, Augustine says (Gen. ad lit. xii, 15): "When the same image that comes into the mind of a speaker presents itself to the mind of the sleeper, so that the latter is unable to distinguish the imaginary from the real union of bodies, the flesh is at once moved, with the result that usually follows such motions; and yet there is as little sin in this as there is in speaking and therefore thinking about such things while one is awake."
I answer that, Nocturnal pollution may be considered in two ways. First, in itself; and thus it has not the character of a sin. For every sin depends on the judgment of reason, since even the first movement of the sensuality has nothing sinful in it, except in so far as it can be suppressed by reason; wherefore in the absence of reason's judgment, there is no sin in it. Now during sleep reason has not a free judgment. For there is no one who while sleeping does not regard some of the images formed by his imagination as though they were real, as stated above in I, 84, 8, ad 2. Wherefore what a man does while he sleeps and is deprived of reason's judgment, is not imputed to him as a sin, as neither are the actions of a maniac or an imbecile.
Secondly, nocturnal pollution may be considered with reference to its cause. This may be threefold. One is a bodily cause. For when there is excess of seminal humor in the body, or when the humor is disintegrated either through overheating of the body or some other disturbance, the sleeper dreams things that are connected with the discharge of this excessive or disintegrated humor: the same thing happens when nature is cumbered with other superfluities, so that phantasms relating to the discharge of those superfluities are formed in the imagination. Accordingly if this excess of humor be due to a sinful cause (for instance excessive eating or drinking), nocturnal pollution has the character of sin from its cause: whereas if the excess or disintegration of these superfluities be not due to a sinful cause, nocturnal pollution is not sinful, neither in itself nor in its cause.
A second cause of nocturnal pollution is on the part of the soul and the inner man: for instance when it happens to the sleeper on account of some previous thought. For the thought which preceded while he was awake, is sometimes purely speculative, for instance when one thinks about the sins of the flesh for the purpose of discussion; while sometimes it is accompanied by a certain emotion either of concupiscence or of abhorrence. Now nocturnal pollution is more apt to arise from thinking about carnal sins with concupiscence for such pleasures, because this leaves its trace and inclination in the soul, so that the sleeper is more easily led in his imagination to consent to acts productive of pollution. In this sense the Philosopher says (Ethic. i, 13) that "in so far as certain movements in some degree pass" from the waking state to the state of sleep, "the dreams of good men are better than those of any other people": and Augustine says (Gen. ad lit. xii, 15) that "even during sleep, the soul may have conspicuous merit on account of its good disposition." Thus it is evident that nocturnal pollution may be sinful on the part of its cause. on the other hand, it may happen that nocturnal pollution ensues after thoughts about carnal acts, though they were speculative, or accompanied by abhorrence, and then it is not sinful, neither in itself nor in its cause.
The third cause is spiritual and external; for instance when by the work of a devil the sleeper's phantasms are disturbed so as to induce the aforesaid result. Sometimes this is associated with a previous sin, namely the neglect to guard against the wiles of the devil. Hence the words of the hymn at even: "Our enemy repress, that so our bodies no uncleanness know" [Translation W. K. Blount].
On the other hand, this may occur without any fault on man's part, and through the wickedness of the devil alone. Thus we read in the Collationes Patrum (Coll. xxii, 6) of a man who was ever wont to suffer from nocturnal pollution on festivals, and that the devil brought this about in order to prevent him from receiving Holy Communion. Hence it is manifest that nocturnal pollution is never a sin, but is sometimes the result of a previous sin.
Reply to Objection 1. Solomon did not merit to receive wisdom from God while he was asleep. He received it in token of his previous desire. It is for this reason that his petition is stated to have been pleasing to God (1 Kings 3:10), as Augustine observes (Gen. ad lit. xii, 15).
Reply to Objection 2. The use of reason is more or less hindered in sleep, according as the inner sensitive powers are more or less overcome by sleep, on account of the violence or attenuation of the evaporations. Nevertheless it is always hindered somewhat, so as to be unable to elicit a judgment altogether free, as stated in I, 84, 8, ad 2. Therefore what it does then is not imputed to it as a sin.
Reply to Objection 3. Reason's apprehension is not hindered during sleep to the same extent as its judgment, for this is accomplished by reason turning to sensible objects, which are the first principles of human thought. Hence nothing hinders man's reason during sleep from apprehending anew something arising out of the traces left by his previous thoughts and phantasms presented to him, or again through Divine revelation, or the interference of a good or bad angel.
Source: Summa Theologica (New Advent)