St. Bede the Venerable and St. Aurelius Augustine explain Christ uses the term "sleeping" because He as God could raise people again, while St. John Chrysostom teaches He thus shows it easy for Him to raise the dead and that we should not be afraid of dying. Further Saint Paul the Apostle calls (1. Cor 15:20) the risen Christ "the first fruits of all those who have fallen asleep" which St. Thomas Aquinas explains to mean the dead in hope of Resurrection.
Catena Aurea Matthew 9:23-26:
Bede, in Luc.: As though He had said, To you she is dead, but to God who has power to give life, she sleeps only both in soul and body.
Chrys.: By this saying, He soothes the minds of those that were present, and shews that it is easy to Him to raise the dead; the like He did in the case [p. 352] of Lazarus, “Our friend Lazarus sleepeth.” [John 11:11] This was also a lesson to them not to be afraid of death; forasmuch as He himself also should die, He made His disciples learn in the persons of others confidence and patient endurance of death. For when He was near, death was but as sleep.
Catena Aurea John 11:11-16:
AUG. It was really true that He was sleeping. To our Lord, he was sleeping; to men who could not raise him again, he was dead. Our Lord awoke him with as much ease from his grave, as you awake a sleeper from his bed. He calls him then asleep, with reference to His own power, as the Apostle says, But 1 would not have you to be ignorant, concerning them which are asleep. Asleep, He says, because He is speaking of their resurrection which was to be. But as it matters to those who sleep and wake again daily, what they see in their sleep, some having pleasant dreams, others painful ones, so it is in death; every one sleeps and rises again with his own account.
St. Thomas Aquinas on 1. Corinthians 15:20-28:
The first fruits, I say, of those who have fallen asleep, i.e., of the dead who rest in hope of the resurrection. From this can be inferred the conditional statement previously made, because we say and it is true, if Christ Who is the first fruit of those that sleep, arose, then also all others asleep.