Saint Diadochos of Photiki. "On Spiritual Knowledge and Discrimination. One Hundred Texts". From Philokalia, Vol. 1.
31. When our intellect begins to perceive the grace of the Holy Spirit, then Satan, too, importunes the soul with a sense of deceptive sweetness in the quiet times of the night, when we fall into a light kind of sleep. If the intellect at that time cleaves fervently to the remembrance of the glorious and holy name of the Lord Jesus and uses it as a weapon against Satan's deception, he gives up this trick and for the future will attack the soul directly and personally. As a result the intellect clearly discerns the deception of the evil one and advances even further in the art of discrimination.
32. The experience of true grace comes to us when the body is awake or else on the point of falling asleep, while in fervent remembrance of God we are welded to His love. But the illusion of grace comes to us, as I have said, when we fall into a light sleep while our remembrance of God is half-hearted. True grace, since its source is God, gladdens us consciously and impels us towards love with great rapture of soul. The illusion of grace, on the other hand, tends to shake the soul with the winds of deceit; for when the intellect is strong in the remembrance of God, the devil tries to rob it of its experience of spiritual perception by taking advantage of the body's need for sleep. If the intellect at that time is remembering the Lord Jesus attentively, it easily destroys the enemy's seductive sweetness and advances joyfully to do battle with him, armed not only with grace but also with a second weapon, the confidence gained from its own experience.
33. Sometimes the soul is kindled into love for God and, free from all fantasy and image, moves untroubled by doubt towards Him; and it draws, as it were, the body with it into the depths of that ineffable love. This may occur when the person is awake or else beginning to fall asleep under the influence of God's grace, in the way I have explained. At the same time, the soul is aware of nothing except what it is moving towards. When we experience things in this manner, we can be sure that it is the energy of the Holy Spirit within us. For when the soul is completely permeated with that ineffable sweetness, at that moment it can think of nothing else, since it rejoices with uninterrupted joy. But if at that moment the intellect conceives any doubt or unclean thought, and if this continues in spite of the fact that the intellect calls on the holy name -not now simply out of love for God, but in order to repel the evil one - then it should realize that the sweetness it experiences is an illusion of grace, coming from the deceiver with a counterfeit joy. Through this joy, amorphous and disordered, the devil tries to lead the soul into an adulterous union with himself. For when he sees the intellect unreservedly proud of its own experience of spiritual perception, he entices the soul by means of certain plausible illusions of grace, so that it is seduced by that dank and debilitating sweetness and fails to notice its intercourse with the deceiver. From all this we can distinguish between the Spirit of truth and the spirit of error. It is impossible, however, for someone consciously to taste the divine goodness or consciously to realize when he is experiencing the bitterness of the demons, unless he first knows with assurance that grace dwells in the depths of his intellect, while the wicked spirits cluster round only the outside of the heart. This is just what the demons do not want us to know, for fear that our intellect, once definitely aware of it, will arm itself against them with the remembrance of God.