You may not have heard of the Zeigarnik Effect, but you’ve almost certainly experienced it. If you’ve ever lain in bed late at night mulling over the day’s unfinished business, or been unable to shake an earworm you heard a snippet of many hours ago, you’re no stranger to this psychological phenomenon – the tendency for us to remember unfinished tasks better than completed ones.

The effect is named after a Lithuanian psychologist, Bluma Zeigarnik. Sitting in a Viennese café in the late 1920s, she noticed something peculiar: her waiter had an uncanny ability to remember customers’ orders in perfect detail – right up until the bills were paid.

Back in the lab, Zeigarnik decided to investigate further. She set a group of children and adults some simple tasks like placing beads on a string, putting together puzzles, or solving math problems, and after these activities and an hour-long delay to clear the mind, she asked the participants to describe what they had been doing. Simple.

But here’s the twist: only half of them were allowed to finish their task. And Zeigarnik found the same effect as she had noticed in that café: people who had been interrupted were twice as likely to remember what they had been doing compared to those who had finished.

So what’s behind the effect? Zeigarnik herself explained the phenomenon as a manifestation of “psychic tension” – a term that owes rather a lot to her training in Gestalt psychology and the fact that it was the 1920s. Today, psychologists think it comes down to the way our brain processes memory.

“The Zeigarnik effect reveals a great deal about how memory works,” explains Kendra Cherry for VeryWellMind.

“Once information is perceived, it is often stored in sensory memory for a very brief time. When we pay attention to information, it moves into short-term memory,” she writes. “Many of these short-term memories are forgotten fairly quickly, but through the process of active rehearsal, some of this information is able to move into long-term memory.”

In other words, the Zeigarnik Effect is kind of an involuntary mental to-do list: tasks that are unfinished stay in our mind, constantly occupying our short-term memory because we keep refreshing the timer on it. Once those tasks are complete, though, they get crossed out – we stop going back to them, so our short-term memory just lets them go.

So now we know what’s going on, we can use it to our advantage.

“Procrastination bites worst when we’re faced with a large task that we’re trying to avoid starting,” wrote Jeremy Dean, a researcher in psychology and author of PsyBlog.

“It might be because we don’t know how to start or even where to start,” he said. “What the Zeigarnik effect teaches is that one weapon for beating procrastination is starting somewhere … anywhere.

“Don’t start with the hardest bit, try something easy first. If you can just get under way with any part of a project, then the rest will tend to follow,” he wrote.

Research into the Zeigarnik effect has also shown that it’s stronger under certain circumstances: we’re better at returning to finish tasks when we know what needs to be done. As Ernest Hemingway once advised: “The best way [to write] is always to stop when you are going good and when you know what will happen next. If you do that every day when you are writing a novel, you will never be stuck.”

The Zeigarnik effect can be particularly useful when studying. Scheduling tactical breaks in your study time may help you recall information later on: “Many students might think it’s best to cram right before their exams, trying to absorb as much information as possible in a small amount of time,” wrote Stephanie Wright for PsychCentral. “However, the Zeigarnik effect says it may be better to break up your study time into smaller sessions over a longer period.”

But perhaps most helpfully of all, we can exploit the Zeigarnik effect to improve our mental health. When you learn that unfinished tasks will loom large in your mind until they’re complete, you start to realize that each new venture – when taken on before its predecessor is complete – will only add to your mental stress.

So make a to-do list, tackle the easy things first – and use the Zeigarnik effect to its fullest.