You make a really good point. How can a body be asleep when it has been vaporized? The answer is a relatively simple one. The are many times in the Bible where sleep and the concept of sleeping are used as a euphemism for being dead. Some examples are found in 1 Corinthians 15:6, John 11:11, 2 Peter 3:4, Daniel 12:2, and others. So you see, it is not that our bodies are literally asleep, persay, but the concept of sleep is used as a euphemism for being dead. The body waits in the grave or its final resting place (be wherever it may) to be renewed into its eternal state, much as we awaken every night from sleep refreshed and recharged.
Perhaps most importantly is to ask this question: “Why would Jesus, Peter, and Paul refer to death as sleeping?” The answer to that question is the very basis of our faith! For the Christian, death is merely a temporary thing, no more permanent than our nightly sleep! It is true that it is appointed for us to die once (Hebrews 9:27), but the power of the gospel is to be found in the triumphant end of Revelation:
“And I heard a loud voice from the throne saying, ‘Behold, the dwelling place of God is with man. He will dwell with them, and they will be his people, and God himself will be with them as their God. He will wipe away every tear from their eyes, and death shall be no more, neither shall there be mourning, nor crying, nor pain anymore, for the former things have passed away.’ And he who was seated on the throne said, ‘”Behold, I am making all things new. (Revelation 21:3-5, ESV)'”
Those words are what we believe in. That is the final culmination of the age, when we finally are united with God, when the fullness of the promises of scripture is finally brought to fruition. Death can be referred to by these men as sleep because that really is all it is for us!