Unbelievers are in darkness, engulfed in mental, moral, and spiritual darkness because of sin and unbelief.
Believers live in a completely different sphere of life than those who remain in darkness. Because believers have been delivered from the domain of darkness, they are taken out of the night of sin and ignorance and put into the light of God and under the control of the truth. Because Christians are in the light, they should not sleep in spiritual indifference and comfort, but be alert to the spiritual issues around them.
Commentary from the MacArthur study Bible, notes for 1 Thessalonians 5:4,5,6.