R. Douglas Fields writes about the vigorous activity of the brain during sleep:

Midway between our unconscious and conscious minds there is the altered mental state of sleep. If you should live to the age of seventy-five, you will have spent perhaps twenty-five of those years asleep. What goes on in your head during that block of your lifetime is largely beyond your knowledge or comprehension. It is a mysterious and still mystical chunk of ourselves.

If sleep were simply a nightly hibernation, a shutting down of our system in the dark, it could be understood as a reasonable strategy to save power for the daytime when we can be physically active. Sleep might be much like a laptop computer going into temporary hibernation to save resources during long periods of inactivity. But hibernation is hardly what goes on in the human brain during sleep. Sleep is a vigorous period of brain activity. It is an altered state, not an inert state.

There are cycles and patterns of activity during our nocturnal unconscious life shuttling enormous amounts of activity through different brain circuits. Events of the day—conscious and unconscious—are reexamined, sorted, associated, filed, or discarded. Memories are moved from one place in the brain and filed in different places in our cerebral cortex according to such factors as the type of information they contain, their connections to other events, and the internal emotional states of mind stamping them with significance.

Possible Preaching Angle:

We read repeatedly in Scripture that God spoke to one servant or another in a dream, or while they slept. He even spoke to unbelievers whose actions could impact his people. Why does God choose to speak to people while they sleep? Maybe it is because they are so busy or so distracted or so obstinate while awake that he speaks to them when they are asleep.