John Gill Commentary Psalms 139:11

"If I say, Surely the darkness shall overwhelm me, And the light about me shall be night;" — Psalms 139:11 (ASV)

If I say, surely the darkness shall cover me
The darkness of a cloud or of the night, so that my actions shall not be seen; that is, if I entertain such a thought in my mind, that what I do in the dark will escape the sight and knowledge of God, and so be emboldened to commit it;

even the night shall be light about me ;
and make all my works manifest, as light does.