The Zeigarnik effect, named after psychologist Bluma Zeigarnik, describes the phenomenon where people remember unfinished or interrupted tasks better than completed ones. This effect, observed by Zeigarnik in 1927, suggests that incomplete tasks create a cognitive tension that interferes with memory retention until the task is completed.
In other words, your to do list takes up a lot of brain bandwidth.
According to Sonke Ahrens, in the excellent How to Take Smart Notes in Zeigarnik’s follow-up research, we learned that we don’t actually have to finish tasks to convince our brains to stop thinking about them. All we have to do is to write them down in a way that convinces our brains that it will be taken care of, and that reduces the amount of bandwidth it takes up. “That’s right: The brain doesn’t distinguish between an actual finished task and one that is postponed by taking a note. By writing something down, we literally get it out of our heads.”