Complex sentence: While the conscious mind sleeps and the unconscious mind works to clean it up, sleep also frees your subconscious to work solely on problems which it was too distracted to fully concentrate on during the day.
Split into simpler sentences: While the conscious mind sleeps, the unconscious mind works to clean it up. During this time, sleep frees your subconscious to work solely on problems which it was too distracted to fully concentrate on during the day.