When you go into REM sleep, your body stops releasing certain neurotransmitters thus causing a drop in vigilance (am I in danger?) and mood-modulation (so what?). Also, the prefrontal cortex becomes less active causing a diminished sense of logic (what’s going on?) and planning (what do I do next?).
You notice how the physical state of sleep denotes all these low cognitive functions. That is to say, a state of sleep means low mental processing. Sleep is a diminished level of mental exertion. So you see how it is you can be asleep even while you’re awake. You can choose to exercise your mind…to wake up!
Consider the following texts with this concept of what it means to sleep in mind:
Proverbs 6:9-11 “How long wilt thou sleep, O sluggard? when wilt thou arise out of thy sleep? Yet a little sleep, a little slumber, a little folding of the hands to sleep: So shall thy poverty come as one that travelleth, and thy want as an armed man.”
Matthew 25 “Then shall the kingdom of heaven be likened unto ten virgins, which took their lamps, and went forth to meet the bridegroom. And five of them were wise, and five were foolish…While the bridegroom tarried, they all slumbered and slept.”
Romans 13:11-12 “And that, knowing the time, that now it is high time to awake out of sleep: for now is our salvation nearer than when we believed. The night is far spent, the day is at hand: let us therefore cast off the works of darkness, and let us put on the armour of light.”
1 Thessalonians 5 “But of the times and the seasons, brethren, ye have no need that I write unto you. For yourselves know perfectly that the day of the Lord so cometh as a thief in the night…But ye, brethren, are not in darkness, that that day should overtake you as a thief…Therefore let us not sleep, as do others; but let us watch and be sober. For they that sleep sleep in the night; and they that be drunken are drunken in the night. But let us, who are of the day, be sober, putting on the breastplate of faith and love; and for an helmet, the hope of salvation.”
Psalm 121:1-4 “I will lift up mine eyes unto the hills, from whence cometh my help. My help cometh from the Lord, which made heaven and earth. He will not suffer thy foot to be moved: he that keepeth thee will not slumber. Behold, he that keepeth Israel shall neither slumber nor sleep.”