I will bless the Lord, who hath given his counsel. God has become David's "Counsellor" (see Psalms 32:8), makes suggestions to him which he follows, and so guides his life that he feels bound to praise and bless him for it.
My reins also instruct me in the night seasons. The reins, according to Hebrew ideas, are the seat of feeling and emotion. David is "instructed" or "stimulated" (Hengstenberg) to bless God by the feelings which stir within him as he lies awake at night—feelings, we must suppose, of affection and gratitude.