It is not a normal "sleep," of course. It is a sleep in which there is no thought, brain activity or life whatsoever. Passages throughout the Bible show this to be the case.
For example, Ecclesiastes 9 states, "For the living know that they will die; but the dead know nothing . . . For there is no work or device or knowledge or wisdom in the grave where you are going" (Ecclesiastes 5-10).
Daniel 12:2 Daniel 12:2And many of them that sleep in the dust of the earth shall awake, some to everlasting life, and some to shame and everlasting contempt.
American King James Version×describes the dead as "those who sleep in the dust of the earth," who later "shall awake" through being resurrected.
Job speaks of the state of the dead on more than one occasion. "Why did I not die at birth? Why did I not perish when I came from the womb?...For now I would have lain still and been quiet, I would have been asleep; then I would have been at rest...There the wicked cease from troubling, and there the weary are at rest" (Job 3:11-17 Job 3:11-17  Why died I not from the womb? why did I not give up the ghost when I came out of the belly?
 Why did the knees prevent me? or why the breasts that I should suck?
 For now should I have lain still and been quiet, I should have slept: then had I been at rest,
 With kings and counsellors of the earth, which build desolate places for themselves;
 Or with princes that had gold, who filled their houses with silver:
 Or as an hidden untimely birth I had not been; as infants which never saw light.
 There the wicked cease from troubling; and there the weary be at rest.
American King James Version×).
Many centuries later the biblical account of the death of Lazarus, a friend of Jesus, illustrates death to be a sleeplike state. "Now a certain man was sick, Lazarus of Bethany" (John 11:1 John 11:1Now a certain man was sick, named Lazarus, of Bethany, the town of Mary and her sister Martha.
American King James Version×). Jesus decided to go to him, but, so He could perform a miracle to strengthen His disciples' faith, He waited until Lazarus died.
Before going to Bethany, Jesus discussed the condition of Lazarus with His disciples. He told them Lazarus was asleep and that He was going to awaken him (John 11:11-14 John 11:11-14  These things said he: and after that he said to them, Our friend Lazarus sleeps; but I go, that I may awake him out of sleep.
 Then said his disciples, Lord, if he sleep, he shall do well.
 However, Jesus spoke of his death: but they thought that he had spoken of taking of rest in sleep.
 Then said Jesus to them plainly, Lazarus is dead.
American King James Version×). The disciples responded that sleep was good because it would help him get well (John 11:12). Jesus then plainly told them, "Lazarus is dead" (John 11:14 John 11:14Then said Jesus to them plainly, Lazarus is dead.
American King James Version×). Notice that Jesus stated emphatically that Lazarus was dead, but at the same time He described death as a condition like sleep.
When the time came for Jesus to act, "He cried with a loud voice, 'Lazarus, come forth!' And he who had died came out bound hand and foot with graveclothes...Jesus said to them, 'Loose him, and let him go'" (John 11:43-44 John 11:43-44  And when he thus had spoken, he cried with a loud voice, Lazarus, come forth.
 And he that was dead came forth, bound hand and foot with grave clothes: and his face was bound about with a napkin. Jesus said to them, Loose him, and let him go.
American King James Version×).
Lazarus had not gone to heaven or hell. He had been entombed, where he "slept" in death until Jesus called him out of the grave by a miraculous resurrection.
Like Lazarus, everyone enters a figurative state of sleep at death. The dead are unconscious. The common belief is that at death the body goes to the grave and the soul remains conscious and goes either to heaven or hell. Yet as we have seen, this belief is not biblical.
In another reference that describes the state of the dead, Paul refers to the righteous dead who will be resurrected to meet Christ in the air as being "asleep":
"For this we say to you by the word of the Lord, that we who are alive and remain until the coming of the Lord will by no means precede those who are asleep. For the Lord Himself will descend from heaven with a shout, with the voice of an archangel, and with the trumpet of God. And the dead in Christ will rise first. Then we who are alive and remain shall be caught up together with them in the clouds to meet the Lord in the air. And thus we shall always be with the Lord" (1 Thessalonians 4:15-17 1 Thessalonians 4:15-17  For this we say to you by the word of the Lord, that we which are alive and remain to the coming of the Lord shall not prevent them which are asleep.
 For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first:
 Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord.
American King James Version×).
So those who are in their graves will be resurrected, rising to meet the returning Messiah along with His followers who are then still alive. They all will be caught up in the air to meet Christ in the first resurrection. They will then return to the earth to reign with Him in the Kingdom of God.
That the dead are figuratively in a state of sleep, awaiting the resurrection, "was the prevalent opinion until as late as the 5th century" (D.P. Walker, The Decline of Hell: Seventeenth-Century Discussions of Eternal Torment, 1964, p. 35). The change away from the biblical teaching occurred several centuries after Christ. The plain teaching of the Bible is that the dead are unconscious, waiting in the grave. They are, as Jesus and Paul put it, sleeping. They will not awake until the resurrection.
Even Martin Luther, leader of the Protestant Reformation, wrote at one point: "It is probable, in my opinion, that, with very few exceptions indeed, the dead sleep in utter insensibility till the day of judgment . . . On what authority can it be said that the souls of the dead may not sleep . . . in the same way that the living pass in profound slumber the interval between their downlying at night and their uprising in the morning?" (Letter to Nicholas Amsdorf, Jan. 13, 1522, quoted in Jules Michelet, The Life of Luther, translated by William Hazlitt, 1862, p. 133). Yet the Reformation did not embrace the truth that the dead sleep in total unawareness.
Eventually all will arise from this sleep. As Jesus said, the hour is coming "in which all who are in the graves will hear His voice and come forth" (John 5:28-29 John 5:28-29  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, to the resurrection of life; and they that have done evil, to the resurrection of damnation.
American King James Version×). This is the comforting and encouraging truth revealed in the Scriptures.