Death is not the end, however. There is to be a resurrection from death. Everyone who has died will experience a resurrection and will live again. John 5:28,29 tells us: “Marvel not at this: for the hour is coming, in the which all that are in the graves shall hear his voice, And shall come forth; they that have done good, unto the resurrection of life; and they that have done evil, unto the resurrection of damnation.” The resurrection predicted in the Bible will be totally unlike the reported experiences of people who believe they have died and come back.
 

"The Bible tells us in Ecclesiastes 9:5,6,10 that “the living know that they shall die: but the dead know not any thing..."

Basing Biblical doctrines primarily from that written in the book of Ecclesiastes is dodgy ground because the book of Ecclesiastes is written from the point of view of  mere human reason - ie as a philosopher, one 'under the sun' (a phrase that occurs 29 times in the book.). As such, there are things in it that may seem true to 'one under the sun', but are statements that rely on human reasoning, not God's. Ecc 9:5 would certainly be a scripture that I put in this group. William MacDonald, in his Bible Believers Commentary, writes the following concerning Ecc 9:5 -

"This verse is constantly used by false teachers to prove that the soul sleeps in death, that consciousness ceases when the last breath is taken. Bit it is senseless to build a doctrine pf the hereafter on this verse, or on this book, for that matter. Ecclesiastes represents man's best conclusions as he searches for answers "under the sun"... It is what a wise man might think if he did not have a Bible."

Contrary to what human wisdom would say about men's thoughts after death, the Bible is full of examples that show that people are conscious after death as Jesus, Paul, and John in the book of Revelation show us. But more on that later!  

Also in Psalms 146:4 we see that in death, “His breath goeth forth, he returneth to his earth; in that very day his thoughts perish.”

While some translations, such as the KJV use the word 'thought' in this verse as you have quoted above, most do not. And I'm not just talking about modern 'loose' translations. For example:

NKJV 'His spirit departs, he returns to his earth; In that very day his plans perish.'
RSV 'When his breath departs he returns to his earth; on that very day his plans perish.'
NASB 'His spirit departs, he returns to the earth; In that very day his thoughts perish.'
J.N. Darby Translation 'His breath goeth forth, he returneth to his earth; in that very day his purposes perish.'
Young 'His spirit goeth forth, he returneth to his earth, In that day have his thoughts perished.'
NIV 'When their spirit departs, they return to the ground; on that very day their plans come to nothing.'

So I wouldn't go basing a doctrine of soul sleep on that verse either as most versions indicate that it is the persons plans and purposes that they have been thinking about which perish and come to nothing upon death.

There is a verse in the Bible that apparently contradicts what we have just learned about death...Luke 23:42,43... seems to contradict the other verses showing death to be like a very deep sleep, without consciousness.

A quick note on this... Maybe it is just me, but I find that quite misleading. Well, at best misleading, at worst deceptive. Seems like the author wants to indicate that there is only one verse that seemingly contradicts what he has just put forth. There are plenty of verses that would teach the complete opposite of what he has written above. And we don't have to go to a book written as one 'under the sun' to prove it. We can look at what Jesus and Paul specifically taught concerning the issue. I'll address these later.

However, consider John 20:17, where Jesus, talking with Mary on Sunday morning after His resurrection, said, “Touch me not; for I am not yet ascended to my Father: But go to my brethren, and say unto them, I ascend unto my Father, and your Father; and to my God, and your God.” So, on Sunday morning Jesus made the statement that He had not yet been to paradise, to God the Father. How, then, could He have told the thief on the cross that he would be in paradise on Friday evening with Jesus when He Himself would not be there? Where was Jesus after He died on the cross? He was asleep in the grave, just as the Scriptures say, until His resurrection on Sunday morning. Jesus Himself said in Matthew 12:40 that He would remain in the grave until his resurrection: “For as Jonas was three days and three nights in the whale’s belly; so shall the Son of man be three days and three nights in the heart of the earth.”

It is true that Jesus had not ascended to his Father by the time of his resurrection on Sunday morning as John 20:17 above indicates. It is a big assumption that the promise of 'paradise', at that time, to the thief on the cross, meant Heaven and being in the Father's presence. I don't believe that this is the case at all. Up until Jesus' resurrection, Jesus taught that those who died went either to Hades or to Abraham's bosom and their was a gulf between them. (Luke 16:19-31) Upon death, Jesus and the thief who believed in Jesus upon the cross went to Abraham's bosom (paradise). Contrary to what is written above, the scriptures do not state that Jesus was 'asleep in the grave'. As we have just seen, Jesus taught that both those who went to Hades or Abrahams bosom were very much "conscious" and aware of there surroundings. And it is crazy to suggest that Jesus, who is God, would have been experiencing 'soul sleep' during this time. Upon his resurrection, we read that many other graves where opened and people were resurrected and the Bible seems to indicate that those in Abraham's bosom where taken to heaven at his ascension (Eph 4:7-9) Today, after Jesus' resurrection and ascension, 'Paradise' is in Heaven as Paul indicates in 2 Cor 12:1-4. There is no contradiction here apart from that which is in the author's mind.

Where, then, is the problem? Since Luke 23:43 is the verse that stands in contradiction with the other verses we have considered, there must be some problem with this one. The difficulty can be resolved easily. The original Greek text was written without punctuation. Punctuation was added later by the translators. The problem with this verse is the placement of the comma before the words “To day.” If the comma is placed after the words “To day,” the verse says that Jesus on that day promised the thief that he would be with Him in paradise after the resurrection. This verse would then agree with the other verses of Scripture concerning death. It should therefore read, “Verily I say unto thee today, that thou shalt be with me in Paradise.” Jesus promised the thief “today” a place in paradise after the resurrection. Remember that the entire Bible must agree with itself; this is one of the methods God has given us so that we might know when we have these difficult verses interpreted correctly. Remember also that the original text had no punctuation and that the problem is with the comma, which was put there by a translator who did not understand.

I agree that the Greek didn't have commas, but what would be the point in Jesus saying 'Truly, I say unto you today, you shall be with me in Paradise." Why even add the word 'today' in that sentence if that is the way we should read it? Of course Jesus was talking to the thief 'today'. Anytime you talk directly to someone, it is today! No, it doesn't seem like a valid argument to me. I looked up on the computer and couldn't find any other time that Jesus said he was telling them something 'today'. It seems redundant and pointless. No, it should read as every version that has ever comes out renders it - "Truly, I say unto you, today you shall be with me in Paradise." A wonderful promise!

I certainly agree that the Bible must agree with itself, as the author wrote above, and that is another reason why I reject soul sleep. There are plenty of scriptures that show that after death, people are still very much conscious of their surroundings even before the resurrection. As already indicated, Jesus specifically taught this in Luke 16:19-31, as well as his promise to the thief on the cross. Paul clearly believed it for he wrote:

2 Cor 5:6-8 'Therefore we are always confident and know that as long as we are at home in the body we are away from the Lord. We live by faith, not by sight. We are confident, I say, and would prefer to be away from the body and at home with the Lord.' 

Phil 1:21-24 'For to me, to live is Christ and to die is gain. If I am to go on living in the body, this will mean fruitful labor for me. Yet what shall I choose? I do not know! I am torn between the two: I desire to depart and be with Christ, which is better by far; but it is more necessary for you that I remain in the body.'

1 Thes 4:13-14 'Brothers, we do not want you to be ignorant about those who fall asleep, or to grieve like the rest of men, who have no hope. We believe that Jesus died and rose again and so we believe that God will bring with Jesus those who have fallen asleep in him.'

You will note above that when Jesus returns from Heaven, to earth, at His return, he will bring with him those that have died in Christ. This indicates, as Paul did in the verses earlier, that they are in Heaven at the moment and it is from their that He will bring them. This agrees with what is written in Heb 12:23 which specifically states that the spirits of the righteous and the church (those that have died in Christ) are currently in Heaven -

Heb 12:23 'But you have come to Mount Zion, to the heavenly Jerusalem, the city of the living God. You have come to thousands upon thousands of angels in joyful assembly, to the church of the firstborn, whose names are written in heaven. You have come to God, the judge of all men, to the spirits of righteous men made perfect, to Jesus the mediator of a new covenant.'

This agrees exactly with what we see in the book of Revelation. Revelation gives us several glimpses into Heavenly scenes and what we see is that people are there who have died in the tribulation and are awaiting the resurrection (mentioned in Chapter 20). These people are conscious, they talk, sing, worship and are in joy! They are not in a state of soul sleep even though their resurrection hasn't occurred yet. Some examples:

Rev 6:9-11 "When he opened the fifth seal, I saw under the altar the souls of those who had been slain because of the word of God and the testimony they had maintained. They called out in a loud voice, “How long, Sovereign Lord, holy and true, until you judge the inhabitants of the earth and avenge our blood?”  Then each of them was given a white robe, and they were told to wait a little longer, until the number of their fellow servants and brothers who were to be killed as they had been was completed."

Rev 7:9-17 'After this I looked and there before me was a great multitude that no one could count, from every nation, tribe, people and language, standing before the throne and in front of the Lamb. They were wearing white robes and were holding palm branches in their hands. And they cried out in a loud voice:

“Salvation belongs to our God,
who sits on the throne,
and to the Lamb.”

...Then one of the elders asked me, “These in white robes—who are they, and where did they come from?”
I answered, “Sir, you know.” And he said, “These are they who have come out of the great tribulation; they have washed their robes and made them white in the blood of the Lamb. Therefore,

“they are before the throne of God
and serve him day and night in his temple;
and he who sits on the throne will spread his tent over them.
Never again will they hunger;
never again will they thirst.
The sun will not beat upon them,
nor any scorching heat.
For the Lamb at the center of the throne will be their shepherd;
he will lead them to springs of living water.
And God will wipe away every tear from their eyes.”

It seems very clear to me that believers who die go straight to be with the Lord and that 'soul sleep' is a fraud.

Hope this helps.