I have read about soul sleep and it is very interesting. The problem for me concerning it is that it requires some biblical manipulation and goes against the vast history of orthodox christianity.
Paul says in 2 Corinthians 5
6So we are always of good courage. We know that while we are at home in the body we are away from the Lord, 7for we walk by faith, not by sight. 8Yes, we are of good courage, and we would rather be away from the body and at home with the Lord. 9So whether we are at home or away, we make it our aim to please him. 10For we must all appear before the judgment seat of Christ, so that each one may receive what is due for what he has done in the body, whether good or evil.
Also, the thief on the cross sheds some life on this as well.
39One of the criminals who were hanged railed at him,[d] saying, "Are you not the Christ? Save yourself and us!" 40But the other rebuked him, saying, "Do you not fear God, since you are under the same sentence of condemnation? 41And we indeed justly, for we are receiving the due reward of our deeds; but this man has done nothing wrong." 42And he said, "Jesus, remember me when you come into your kingdom." 43And he said to him, "Truly, I say to you, today you will be with me in Paradise."
Soul sleep advocates claim that Jesus actually said, "Truly I say to you today, you will be with me in Paradise." They insist that Jesus never gave him the impression that on that day the thief will be with Christ in Heaven. (As an aside, I did always wonder how Christ would be in Heaven with the thief that day even before his ascension. If someone has an answer for this I would appreciate hearing it.)
But I find it hard to believe that Jesus, in physical and spiritual agony on the cross, would take the time to let the thief know that he was speaking to him 'Today'.