Only in sleep, where there's nothing but mind, can the mind clearly process all of the day's experiences/memories - without distraction. And, perhaps, only in sleep, where there's nothing but mind, can the mind truly understand the meaning of these memories, as well, and assimilate them with all the other memories you've accumulated over time, forming greater meanings - unintelligible in the light of day - building, perhaps, to some ultimate meaning at the culmination of life - unintelligible in the light of living.