‘But you, brothers and sisters, are not in darkness that that day should overtake you as a thief.'
The day overtakes these people because they are in darkness. They are asleep in the wrong sense. But Christians, who are no longer under the power of darkness (Colossians 1:13), nor walking in darkness (John 8:12; 1 John 1:6) should be watching, that is living their lives in the light of His coming, and therefore will not be caught out.