Jesus said to the thief on the cross "today, you will be with me in paradise".
Paradise is another name for Abraham's bosom - recall the story of the rich man and the beggar Lazarus, how the angels took Lazarus to Abraham's bosom, when he died.
Paradise/Abraham's bosom is an angelic realm for the dead who are spiritually righteous.
The word "sleep" is not literal sleep, as in unconsciousness; It's just a metaphorical way of saying rest, or relaxation; you are at peace, or at rest in this realm.
Moses and Elijah appeared on the mount of transfiguration. They were conscious, not asleep. They entered the Earthly realm from paradise/Abraham's bosom, so that they could talk with Jesus.
Also, when Jesus was upon the cross, the pharisees thought that Jesus, when he said "Eli, Eli...", was calling upon Elijah to help him. If Elijah was unconscious, in soul sleep, then he would not have heard Jesus calling to him.
Is it possible that Moses and Elijah were only temporarily awake on the mountain and that once their conversation was over, they returned to an unconscious state. Yeah, that's theroretically possible; but once you consider the story of the rich man and the beggar Lazarus - and what Jesus said to the thief on the cross - the soul sleep theory becomes refuted.
Also, the overwhelming number of near death experience (NDE) accounts of people still remaining conscious after they die, of entering heavenly realms with angels and God, further refutes the soul-sleep theory.
I hope this clarifies things.
God bless and Jesus be with you.
