“One of the things that happens is that during sleep, your brain cleans itself of waste that has accumulated during the day. “During slow-wave sleep, your cerebral spinal fluid channels open up more and remove metabolic waste from your brain,” Roxanne explained to me. Every night, when you go to sleep, your brain is rinsed with a watery fluid. This cerebrospinal fluid washes through your brain, flushing out toxic proteins and carrying them down to your liver to get rid of them. “So when I’m talking to college students, I call this brain-cell poop. If you can’t focus well, it might be you have too much brain-cell poop circulating.” That can explain why, when you are tired, “you get a hung-over sort of feeling”—you are literally clogged up with toxins. This positive kind of brainwashing can only happen when you are asleep.”
―
Johann Hari,
Stolen Focus: Why You Can't Pay Attention—and How to Think Deeply Again