John 11:11-14
11 These things said he: and after that he saith unto them, Our friend Lazarus sleepeth; but I go, that I may awake him out of sleep.
12 Then said his disciples, Lord, if he sleep, he shall do well.
13 Howbeit Jesus spake of his death: but they thought that he had spoken of taking of rest in sleep.
14 Then said Jesus unto them plainly, Lazarus is dead.
come on, it's easy to understand it. the sleep here means dead
In The eyes of Jesus, he really sleep until that day resurrection.
The above doesn't mean that every believe will be dead when the resurrection takes place! There will be living believers at that time who will not have died. And since they are not dead, they will be changed immortal and glorified and will meet those who resurrected, in the clouds. That scripture is just talking about Lazarus who had died and was then resurrected by the Lord. By the way, Lazarus was not resurrected in his immortal and glorified body, for he died again. His spirit is currently in heaven waiting for the resurrection of his body.
Sleep referred to in the scriptures is referring to the body. At the time of death for the believer, their spirit departs from the body and goes to be in the presence of the Lord. When those who are not in Christ die, their spirits depart and go down into Hades where they begin their punishment in torment in flame, as revealed in the event of the rich man and Lazarus.
Jesus is a good example of this. After He died, His spirit departed and went down into Hades. Three days later, His spirit returned to His body and He resurrected.
1-Corinthians 15:50 I declare to you, brothers, that flesh and blood cannot inherit the kingdom of God, nor does the perishable inherit the imperishable.
The very example that you used above is why the living believers must be changed immortal and glorified because this perishable body cannot inherit heaven. Therefore, those who are still alive at the Lord's coming must be changed immortal and glorified.