Revelation 22:5

And there shall be no night there
This is repeated from ( Revelation 21:25 ) to express the certainty of it, and to observe, that the happiness of this state will greatly lie in the light thereof; it will be one everlasting day, (hmera aiwnov) , "day of eternity", or eternal day, as in ( 2 Peter 3:18 )

and they need no candle, nor the light of the sun;
neither artificial nor natural light; neither the dimmer light of the ceremonial law, under the legal dispensation, which was like a candle lighted up in Judea; nor the more clear light of the Gospel and its ordinances, under the present dispensation, which now will be at an end:

for the Lord God giveth them light;
immediately from himself, without the use of means and ordinances; and in his light the saints will see all things clearly; who will be always communicating it to them, and will be their everlasting light; (See Gill on Revelation 21:23).

and they shall reign for ever and ever;
they are made kings now, and in this state they shall reign with Christ for the space of a thousand years; and when they are ended, they shall not cease to reign; nor will Christ, when he delivers up the kingdom to the Father, for his and their kingdom is an everlasting one, ( Revelation 1:6 ) ( 5:10 ) ( 20:4 ) ( Daniel 7:27 ) and here ends the account of this glorious state of things; what follows is the conclusion of the whole book.