Zeigarnik found a quirk of the human mind: When a task is unfinished, we can’t seem to stop thinking about it. We perseverate. Psychologists still argue about why; possibly it’s a kind of constant refresh to keep whatever’s pending from vanishing from our short-term memory, like putting something by the front door at night so you don’t forget to take it with you the next morning.

Whatever the cause, today this is known as the Zeigarnik effect, and psychologists who study task management say it’s part of why so many of us feel perpetually frazzled by the challenge of organizing work and life. When we face all that undone stuff—emails to write, calls to return, people to contact, friends to check in on, memos to draft, children to help—it’s like being a waiter serving a hundred tables at once. If you’ve found yourself in bed at 2 am with your brain screaming at you about that thing you didn’t do, that’s a Zeigarnik moment.