Little Question Six of Question Three
Whence it is that the influx of spirits upon human beings concerns sleep rather than wakefulness
It is doubted why the influx of spirits upon human beings concerns sleep rather or more commonly than wakefulness.
To this it is answered as follows: three distinct actualizations of the interior cognitive power require rest and tranquility of the mind, that is, of the soul—namely, revelation, contemplation, and the investigation of truth.
Since, therefore, sleep is a certain rest of the animal powers, the soul of one sleeping is consequently in a state more apt for revelations.
Hence, because a revelation sent to a human being by God or an angel through a dream is a certain interior address, it would occur in vain when the soul were so distracted, occupied with, or intent upon other things that it did not attend to the spirit addressing it, or did not impress the address upon its memory.
This is manifest also in exterior speech. For no wise person wishing to say something to someone addresses him at a time or place when, because of interior distraction or exterior disturbance, he cannot hear perfectly, or, if he hears, does not receive in his heart what is said.
And therefore the wise man said that speech is not to be poured out where there is no hearing. And in the same place: “Speech occurs in vain where there is no attention, or where there is no understanding of that which is said.”
Good spirits, therefore, since they are most wise and do not wish to address human beings in vain with either interior or exterior speech, reasonably attend at that time to the place, state, and disposition of the person who is to be addressed, so that their addresses may have effect, according to that which is said in Isaiah 55: “My word which shall go forth from my mouth shall not return to me empty.”
From these things it follows that revelation from God or the angels through dreams occurs at a time when the common sense is still bound, because at that time the soul is in complete rest, having no act, and consequently appears to be in a disposition most apt for receiving divine illumination.
These things are from Henry of Hesse in the place cited above.