Albert Barnes Commentary Psalms 149:5

"Let the saints exult in glory: Let them sing for joy upon their beds." — Psalms 149:5 (ASV)

Let the saints be joyful in glory – In the glory of their condition, in the favor of God, in the honor which He bestows on them. Let them rejoice in this; let them shout and triumph over this. Other people rejoice in honor, in wealth, in houses, lands, parks, libraries, and works of art: let the saints rejoice in the glory of being the friends of God, in the hope of heaven. Compare Psalm 84:11.

Let them sing aloud upon their beds – Compare Job 35:10 (note); Acts 16:25 (note); Psalms 34:1 (note). The idea is that in the meditations of the night, when darkness is around them, when alone with God, they may find occasion for exultation and praise. Their hearts may be full of joy, and alone they may give expression to their joy in songs of praise.