Every time any thought intrudes on what you’re doing, you write it down. David Allen, the efficiency expert and author of books, including Getting Things Done, calls this kind of note-taking “clearing the mind.” Writing them down gets them out of your head, clearing your brain of the clutter that is interfering with being able to focus on what you want to focus on. As Allen notes, “Your mind will remind you of all kinds of things when you can do nothing about them, and merely thinking about your concerns does not at all equate to making any progress on them.” Allen noticed that when he made a big list of everything that was on his mind, he felt more relaxed and better able to focus on his work. This observation is based in neurology. When we have something on our minds that is important — especially a To Do item — we’re afraid we’ll forget it, so our brain rehearses it, tossing it around and around in circles in something that cognitive psychologists actually refer to as the rehearsal loop, a network of brain regions that ties together the frontal cortex just behind your eyeballs and the hippocampus in the center of your brain. This rehearsal loop evolved in a world that had no pens and paper, no smartphones or other physical extensions of the human brain; it was all we had for tens of thousands of years and during that time, it became quite effective at remembering things. The problem is that it works too well, keeping items in rehearsal until we attend to them. Writing them down gives both implicit and explicit permission to the rehearsal loop to let them go, to relax its neural circuits so that we can focus on something else. “If an obligation remained recorded only mentally,” Allen says, “some part of me constantly kept thinking that it should be attended to, creating a situation that was inherently stressful and unproductive.” Writing things down conserves the mental energy expended in worrying that you might forget something and in trying not to forget it.”