Nicene and Post-Nicene Fathers, Ser. II, Vol. XI:
The Works of John Cassian.: Chapter XIII. The reason why they are not allowed to go to sleep after the night service.

Chapter XIII.

The reason why they are not allowed to go to sleep after the night service. 705

And these labours they keep up for two reasons, besides this consideration,—that they believe that when they are diligently exerting themselves they are offering to God a sacrifice of the fruit of their hands. And, if we are aiming at perfection; we also ought to observe this with the same diligence. First, lest our envious adversary, jealous of our purity against which he is always plotting, and ceaselessly hostile to us, should by some illusion in a dream pollute the purity which has been gained by the Psalms and prayers of the night: for after that satisfaction which we have offered for our negligence and ignorance, and the absolution implored with profuse sighs in our confession, he anxiously tries, if he finds some time given to repose, to defile us; then above all endeavouring to overthrow and weaken our trust in God when he sees by the purity of our prayers that we are making most fervent efforts towards God: so that sometimes, when he has been unable to injure some the whole night long, he does his utmost to disgrace them in that short hour. Secondly, because, even if no such dreaded illusion of the devil arises, even a pure sleep in the interval produces laziness in the case of the monk who ought soon to wake up; and, bringing on a sluggish torpor in the mind, it dulls his vigour throughout the whole day, and deadens that keenness p. 211 of perception and exhausts that energy 706 of heart which would be capable of keeping us all day long more watchful against all the snares of the enemy and more robust. Wherefore to the Canonical Vigils there are added these private watchings, and they submit to them with the greater care, both in order that the purity which has been gained by Psalms and prayers may not be lost, and also that a more intense carefulness to guard us diligently through the day may be secured beforehand by the meditation of the night.

Footnotes

Missæ.  The use of this word for the offices of the Canonical Hours, though not common, is found also in the Thirtieth Canon of the Council of Agde, a.d. 506. “At the end of the morning and evening missœ, after the hymns, let the little chapters from the Psalms be said.”

Pinguetudo.