A thought experiment: sleeping states are rest states, default states in which the great ocean of brain machinery is repairing itself, humming and chugging along. Don’t ask why do we sleep. Ask why and how we awaken; and then in this being-not-asleep, what function does conscious awareness serve? Consider the dream state and stages of sleep as analogous to the microwave background of the universe, remnants and clues to the big bang, the light that has traveled for billions of years.
Dreams are thoughts and emotions in primitive early form, still and always part of our twenty-four hour existence. Even during our most enlightened and active moments, picture all that basic nighttime brain machinery still pumping along. Our conscious awareness gives the illusion of being a continuous presence, but it really is always coming and going, intermittent, just as dreaming cycles on and off. REM dreams are vivid and active, NREM dreams often more abstract. “Lucid” dreams allow us to go back and forth between apparent sleep/awake states. We can alter dream content with practice. Then there is sleepwalking, night eating binges, sexual behavior (sexsomnia), sleep talking (somniloquy). There are ninety minute cycles superimposed on the beta-gamma-theta-delta-alpha cycles of brain activity. There are daytime lapses, daydreaming, microsleeps, local sleeps of which we are totally unaware.
Sleep and waking and everything connecting these designations are largely not really understood. We are gathering enormous amounts of data but we don’t really have a grasp of how dreams work, how consciousness works, how we can take all this and improve ourselves and reduce human error. The best we know so far, as a prominent researcher is frequently quoted “We sleep because we are tired.” A long way to go; a great opportunity; sleep on it.