Oecumenius

These, therefore, worship God forever; and God dwells among them. One of the saints said that the dwelling of God is the unceasing remembrance of Him that remains in the souls of the saints; therefore, God reasonably dwells there. They worship Him day and night.

Commentary on Revelation ↗

Bede

To be continuously present at the praises of God is not a laborious servitude but a servitude that is pleasant and desirable. "Day and night," indeed, do not exclusively signify the vicissitude of time, but typologically [they signify] its perpetuity.

Homilies on the Gospels 1.10 ↗

Alcuin of York

Therefore they are before the throne, and they serve him day and night in his temple: and he, that sitteth on the throne, dwelleth over them. As if he were saying, "The same people are the throne of God and his temple because we see they are clothed in this kind of garments and have been cleansed by the blood of the Lamb." Note also that he does not say "dwelleth in them" but over them in order to show that they are the throne and the temple at the same time. By day and night he means in success and in adversity.

Commentary On Revelation ↗