The work of the priesthood did not end when the sun went down. The lamps of the Holy Place were to be kept burning all night (Exodus 27:21-22). During the temple period, priests were employed as guards for the house of the Lord, to make sure no one intruded into the sanctuary and went where no one who was not a priest was supposed to go (1 Chronicles 9:22ff). The altar of burnt offering burned night and day with an offering for the atonement of sin – and that fire had to be kept going.
There is deep symbolism here.
God doesn’t sleep. At no time does his eye leave His people (Psalm 34:15). Light in the House of the Lord says he is “up.” His people are always in need of forgiveness – day and night, and God is there to forgive.
And God’s people, who are His servants and (today) priests, should always be vigilant not only in serving Him, but serving those He cares about. Psalm 134 is a prayer of thankfulness for those whose service to God never flags and who are always about the Father’s business.