Is the notion of "soul sleep" true?
According to the Bible we do not possess a soul we are a soul.
Genesis 2:7 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.
1 Corinthians 15:45 And so it is written, The first man Adam was made a living soul;
From this Body + Breath = Living Soul.
It follows that when we cease to breathe we become a dead soul.
The Bible speaks of dead souls but unfortunately the translators hide it by translating the word as ‘body’ instead of ‘soul’.
Genesis 2:7… man became a living soul (Hebrew ‘nephesh’).
Leviticus 21:11 Neither shall he go in to any dead body (Hebrew ‘nephesh’), nor defile himself for his father, or for his mother;
Numbers 6:6 All the days that he separateth himself unto the LORD he shall come at no dead body (Hebrew ‘nephesh’).
Numbers 9:6 And there were certain men, who were defiled by the dead body (Hebrew ‘nephesh’) of a man…
Numbers 19:11 He that toucheth the dead body (Hebrew ‘nephesh’) of any man shall be unclean seven days.
The punishment for sin is the death of the soul.
Ezekiel 18:20 The soul (Hebrew ‘nephesh’) that sinneth, it shall die…
When we die and go to the grave, because we are a soul, our soul dies and goes to the grave.
Job 33:22 Yea, his soul draweth near unto the grave, and his life to the destroyers.
Psalms 49:15 But God will redeem my soul from the power of the grave: for he shall receive me.
Psalms 89:48 What man is he that liveth, and shall not see death? shall he deliver his soul from the hand of the grave?
Psalms 16:10 For thou wilt not leave my soul (Hebrew ‘nephesh’) in hell (Hebrew ‘sheol’ – the grave); neither wilt thou suffer thine Holy One to see corruption.
Acts 2:27 Because thou wilt not leave my soul (Greek ‘psuche’) in hell (Greek ‘hades’ – the grave), neither wilt thou suffer thine Holy One to see corruption… 31 He seeing this before spake of the resurrection of Christ, that his soul was not left in hell, neither his flesh did see corruption. 32 This Jesus hath God raised up, whereof we all are witnesses.
The idea of man possessing an immortal soul (or spirit) which continues on in a conscious existence after death comes from Greek philosophy, not from the word of God.
We, as dead souls, remain in the grave until the resurrection.
May I suggest that you read ‘After Death - What?’ here:
I hope you find this helpful.