To be clear, I do believe in what you call "Soul Sleep", I just wouldn't call it that. The term "soul sleep" is a misnomer because the soul does not sleep. It dies. (this will make more sense when I explain what a "soul" is, a little further down)
In order to answer this question I must first make you understand the meanings of "spirit" and "death". They are in fact polar opposites of each other.
Let's start with the word "spirit". It is a translation of the Greek word pneuma which literally means breath, and when that breath comes from God it is literally the breath of life because God is life. There's a Hebrew word with the exact same meaning used in Genesis 2:7 and it is neshamah.
This is important because it is the distinction between what most people think of when they hear the word "spirit" (a whispy ghostly conscious entity) vs what it actually is, and that is life. And remember that life only comes from God. There is no other source.
If you look up all the places where those words are used (and look up also the word ruach) you'll find that God literally is "the spirit of life" and when he breathed his breath into Adam, his "spirit" entered Adam and he became a living creature (a soul).
And the Lord God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul.
Notice, that man was not given a soul, but he became a soul. Notice also that every living creature on earth is a soul. King Solomon even said that when we die we have no advantage over the beasts for all go to the same place.
For what happens to the children of man and what happens to the beasts is the same; as one dies, so dies the other. They all have the
same breath, and man has no advantage over the beasts, for all is
vanity. All go to one place. All are from the dust, and to dust all
God literally is life and that's why it says when we die, the spirit (the life) returns to God who gave it. (He gave it to us in Genesis 2:7)
the dust returns to the ground it came from, and the spirit (breath of life) returns to God who gave it.
Now that covers the "spirit" of Christ and what happened to it when he died. And make no mistake, he really died. He did not go down into the depths to talk to spirits of the departed as the verse you cite is so often understood. If that were true, then he would not have really died. He would merely have temporarily left his body. That's not death. That's a holiday. Let's be honest. If "death" was simply leaving your body, then that would not be death at all, nor would there be any point in the resurrection, but Christ really did die. His life left him and returned to God who gave it. And if his life returned to God, how could he possibly go talk to sinners? (remember, dead men tell no tales)
So how do I explain the passage of 1 Peter 2:18-20?
Jesus was put to death, but made alive by the Spirit. The NIV says he was made alive in the spirit but I believe a more accurate translation (as in the KJV) is by the spirit.
The reason I believe that is because of what I explained above. This spirit is the breath of God, which made him alive again. The spirit (breath) of God resurrected Jesus. So it was by the life of God that he was made alive. As I said, there is no other source for life.
And by this resurrection he “went and preached unto the spirits in prison”.
The spirits in prison were lives that could not be resurrected to life because they died before Jesus, but by His resurrection now these other lives which had been lost before Him are able to partake in the resurrection to life.
These were not entities in a literal prison. The prison was that from which Jesus has set us all free. The prison of sin. So it is “by the spirit” (ie. by his resurrection - resurrection and spirit literally are the same thing because they both mean life) that the lives which were lost to sin previously, are now able to be freed.
In other words, Peter was merely reassuring believers that even those who had died before Christ have salvation by his resurrection, just like the rest of us. We've all been freed from the same prison.