" We may succeed in provisionally terminating the sum of energy of our waking thoughts by deciding to go to sleep. But we do not always succeed in accomplishing it, or in accomplishing it perfectly. Unsolved problems, harassing cares, overwhelming impressions continue the thinking activity even during sleep, maintaining psychic processes in the system which we have termed the foreconscious. "

― Sigmund Freud , Dream Psychology: Psychoanalysis for Beginners