Ye are the light of the world. A city that is set on an hill cannot be hid. And have no fellowship with the unfruitful works of darkness, but rather reprove them. That ye may be blameless and harmless, the sons of God, without rebuke, in the midst of a crooked and perverse nation, among whom ye shine as lights in the world. The believer is not to sleep, but to watch and be sober.
Sleep refers to spiritual sleep.
When a person is asleep, he isn't alert or involved in what is going on around him. When a believer is spiritually asleep, he is neither alert nor involved in the things of God. His mind and behavior are not concerned with the things of God. He will be caught off guard when the Lord returns. We are not to be like them, unprepared and not watchful.