The Bible mentions a “second death,” thus implying the existence of a first death. The Bible also mentions a “first resurrection,” thus implying a second resurrection. In Revelation 20, it is written,
4 And I saw thrones, and they sat on them, and judgment was committed to them. Then I saw the souls of those who had been beheaded for their witness to Jesus and for the word of God, who had not worshiped the beast or his image, and had not received his mark on their foreheads or on their hands. And they lived and reigned with Christ for a thousand years. 5 (But the rest of the dead did not live again until the thousand years were finished.) This is the first resurrection. 6 Blessed and holy is he who has part in the first resurrection. Over such the second death has no power, but they shall be priests of God and of Christ, and shall reign with Him a thousand years.
In Revelation, before the commencement of the thousand year reign of Christ, there is a resurrection: the first resurrection. Those resurrected are those who were beheaded (and thus died) “for their witness to Jesus and for the word of God.” The author explicitly states that “the rest of the dead did not live again until the thousand years were finished.” However, that these dead did indeed “live again” after the thousand years proves that they, too, are resurrected, albeit at a later time.
The author also states that those who participate in the first resurrection are “blessed and holy” because the second death has no power over them. This implies that the second death has power over those who do not participate in the first resurrection, but in the second resurrection.
Indeed, the prophet Daniel spoke of this second lot of dead when he prophesied,
12:2 And many of those who sleep in the dust of the earth shall awake, some to eternal life, and some to shame and eternal contempt.
Daniel does not state that they all awake simultaneously, but it is as the author of Revelation wrote. Everyone experiences the first death, with few exceptions. It is referred to as a sleep because it is only temporary. Everyone will awake from it, whether at the beginning or end of the thousand years.
Some of those who sleep the first death awake in the first resurrection to eternal life, while the others who do not awake in the first resurrection will awake in the second resurrection, not to eternal life, but to eternal contempt. Over these the second death has power. From this second death, none awake, ever. In that sense, those over whom the second death has power are truly dead, not “sleeping.”