The act of living and staying awake, going about our business, leads to a buildup of toxins in the blood and brain. Cerebrospinal fluid circulates throughout the brain and spinal cord, and it clears away toxins through a series of channels (like waterways) that expand during sleep. Almost none gets cleared away while you are awake. Why all this happens better during sleep than during wakefulness is not fully understood. Too little sleep, or—perhaps counterintuitively—too much sleep, impairs problem solving, attention to detail, memory, motivation, and reasoning.
