The Church's monastic tradition sees as one of the most dangerous enemies of the spiritual life what the psalmist calls "the noonday devil" (Ps 91:6). The monks took this phrase as an apt description of the lethargy or fatigue they battled at about midday.
By that time, they had already risen early for prayers, returned to prayer a number of times, and still had a good chunk of the day ahead of them. They were tired, of course, but not just physically. They were spiritually tired — weary of prayer, exhausted with the things of God. The noonday devil precipitated thoughts of "What's the use? This is pointless." Thus, in the Eastern tradition, Psalm 91 was prayed at midday, to ask for strength against this assault. "Stay awake," Our Lord says (Mt 24:42). And we can hear His words as a warning against the noonday devil. They echo the psalmist's plea for wakefulness: "Consider and answer me, O Lord my God; lighten my eyes, lest I sleep the sleep of death … Open my eyes, that I may behold wondrous things out of thy law" (Ps 13:3; 119:18).
Our Lord and the psalmist both refer not to physical sleep (our bodies need rest at some point) but to that spiritual drowsiness and slumber that deaden us to God's presence, goodness and truth. Once we have fallen asleep spiritually, then all kinds of mischief creep into our souls. Dozing off spiritually brings about a much greater destruction than falling asleep at the wheel.
"Stay awake." Our Lord's words caution us against sloth — that capital vice that brings about spiritual slumber. Sloth is not, as many think, simple laziness (although that is usually a side-effect). Instead, it is a sadness about the good set before us, a boredom with the things of God, a failure to respond with the proper repentance, joy, zeal or love to God's works and goodness. Sloth is a spiritual "ho-hum" or "whatever" in the face of Christ crucified. Once this spiritual languor sets in, we can easily become lazy louts — because we see no reason to make an effort.
But the slothful man can also be tremendously busy. He fills his time with activities to avoid the difficulty of reflection and thought, of facing his sadness with the divine. This describes our culture well. We are constantly on the go, but at the same time bored with the most important things. We rush around in what we call the "real world," as a way of avoiding that most important reality.
We resemble the people in the days of Noah: "they were eating and drinking, marrying and giving in marriage, up to the day that Noah entered the ark. They did not know until the flood came and carried them all away" (Mt 24:38-39). They did not know. They were busy enough, alright, but asleep to the very things that matter. And in the end their slumber undid them.
We do not need to look far to find sloth's damaging effects. Sloth first robs us of the energy needed for the moral life, so we fall into sin more easily. Further, spiritual boredom leads us to seek pleasure elsewhere, in various forms of immorality. If God's works bore us, we will find our happiness in lesser things and make them gods. Sloth also deadens our sense of morality because the drama of good and evil, of right and wrong, tires us. So sloth not only deters us from seeking moral excellence, it also keeps us from perceiving moral truth.
"Stay awake." Our Lord wants us to avoid spiritual slumber. The solution is simple, but not easy. It demands, first, that we fill our minds with thoughts of the divine. The many trivial, worldly thoughts we take in each day leave little room for the supernatural thoughts that inspire us. Accordingly, the battle against sloth demands perseverance in prayer — not just reciting prayers, but mental prayer. We need to reflect deliberately and often on the truths of our faith, the attributes of God, the events of our Lord's life, etc. And when, not if, we grow tired, we need to look to Our Lord and push through the difficulty. We need to remain with Him and not fear the one who lays waste at noon.