And there shall no longer be ANY NIGHT; and they shall not have need of the light of the lamp nor the light of the sun, because THE LORD GOD SHALL ILLUMINE THEM;
This is a reiteration of Revelation 21:23 and Revelation 21:25. The viewpoint in those verses is from the outside of THE CITY. Here we are shown THE CITY's lighting up close and personal.
As with the rest of the Revelation, this, too, has both physical and spiritual meanings. Physically, this verse doesn't say that the present solar system is taken away. The present solar system just isn't needed anymore or may be so dim in comparison to the illumination of the presence of the Lord God. Perhaps we are seeing the full meaning of Jesus' words in John 8:12-20. It also is clear from Jesus' words, that there is a translation to the 'light of life' that will take place. In this verse, here in Revelation, we are shown the 'light of (eternal) life'.
Furthermore, from a spiritual viewpoint, light is the absence of darkness. It is an exercise for the reader to determine what it really means to have darkness removed. Hints are given in Matthew 22:1-13-14, Luke 1:67-79, John 3:16-19-21, Acts 26:1-18-23, 2 Corinthians 6:14-18, Ephesians 5:1-8-20, 1 John 1:5-10, and 1 John 2:7-9-11.
With continual light, we could work forever, but in Hebrews 4:9-10, there is an indication that when we reach THE CITY, we will enter into our REST even as God has entered into His REST. Contrast this with those who are in the lake of fire (Revelation 14:11) or as Jesus referred to it, 'outer darkness' (Matthew 8:5-12-13, Matthew 22:1-13-14, and Matthew 25:14-30. 'Outer darkness' is where non-performing servants are assigned (see verse 3). This place is so horrible that there is weeping and gnashing of teeth. That is a sign of no REST but rather constant turmoil. Every evil and ugly thought and deed is contending with the next evil thought and deed. If you think road rage is bad, try lake of fire rage! This makes me wonder how the cowardly will fare (Revelation 21:8)?
and THEY SHALL REIGN FOREVER AND EVER.
Having just discussed the eternal REST we will have, we still will have a forever obligation to REIGN. This is the bookend to Revelation 1:6. The interesting dilemma, here, however, is that REIGNING implies a 'lower class' or the ones being REIGNED over. Yet the 'lower class' is contained in the 'THEY". In this present world, we cannot understand this duality even as we have a hard time understanding the 'first will be last; and the last, first' principal (Matthew 19:16-30-30).