Sometimes one is unclear about the meaning of a psalm and one can only understand it from one's limited experience. I'm not sure why these servants of God are standing by night in the house of the Lord. It is possible they are on a vigil for a special purpose. It is possible they are sentries on watch-duty. Are they watching for movement from God, movement from God's people, or movement from enemies? Whatever the reason, they are there and it is night. While others are sleeping and at rest, these have given up their slumber.
The psalmist looks at them and considers the work they are doing. Understanding their vocation, he blesses them.
Standing by night is a difficult thing. Others are unaware of ones work. Ones hands grow weary and hang at one's side. But the psalmist commands them to lift their hands and bless the Lord. Then he asks a blessing on their work from God. To bless the Lord in the dark night is a hopeful and powerful thing.
1Behold, bless ye the LORD, all ye servants of the LORD, which by night stand in the house of the LORD.
2Lift up your hands in the sanctuary, and bless the LORD.
3The LORD that made heaven and earth bless thee out of Zion.