Sit down, relax and think of nothing. Struggling? There might be a good reason why your mind seems to wander even when you try very hard to switch off: your brain never really rests. And contrary to popular belief, those idle daydreams might even be beneficial.

For years, neuroscientists worked on the assumption that our brains work hard when given a specific job to do, and switch off when we’re not mentally stimulated. This is why you’ll read about experiments in which volunteers perform a task – tapping a finger, performing some mental arithmetic, looking at evocative pictures – while their brain is scanned. The scan reveals which parts of the brain become more active during the task and which become less active. In this way it is possible to work out how our brain controls our behaviour.  

The big question is: why is the idling brain so active?

Often the neuroscientists want to explore brain activity for a number of different tasks, so they need a way of getting the brain back to a neutral state between tests. This is typically done by asking the person to stare at a simple white cross in the middle of a black screen. By thinking about nothing in particular, the theory goes, the brain should basically switch off.

There is just one problem: it doesn’t.

The first sign that a resting brain is surprisingly active came two decades ago. A student called Bharat Biswal was studying for a PhD at the Medical College of Wisconsin in Milwaukee. He was investigating ways to get a purer signal from a brain scanner, when he noticed that the resting brain isn’t doing nothing. Even when people were told to clear their minds or to stare at a cross, activity in the brain continued. Not only that, the brain scans seemed to reveal this activity was actually coordinated.

Idle network

Then in 1997 an analysis incorporating the results of nine brain scan studies revealed another surprise. Gordon Shulman hoped his analysis would help identify the network that comes to life when people pay attention. But he discovered the opposite – the network which is activated when we do nothing.

It would make sense for the brain to become more active when volunteers shifted from resting to performing a task. Instead, Schulman noticed that some areas of the brain consistently became less active when the resting period ended and the activity began. This suggested that while people were lying quietly in the scanner supposedly doing nothing, parts of their brains were in fact more active than when the volunteers were actively performing a task.

It took a while for the idea that the brain never rests to catch on. For years neuroscientists had thought that brain circuits switched off when they weren’t needed. In 1998 the neuroscientist Marcus Raichle, now one of the leaders in the field, even had a paper rejected by a referee who said the apparent activity must surely be down to an error in the data.

Today things are very different. Almost 3000 scientific papers have been published on the topic of the brain’s surprisingly busy “resting state”. Some object to this term for the very reason that the brain isn’t resting at all. They prefer instead to talk about the “default mode network” – the areas of the brain which remain active while we are apparently idle.

Daydreaming essentially creates memories of events that haven’t happened

How do you rest?

Answer in this global survey

Take part in The Rest Test, a survey of what people around the world think about rest. Which activities do you find restful and do you get enough time to rest? How much does the mind wander when you are resting?

The survey is part of a Wellcome Collection collaboration with BBC Radio 4. Claudia Hammond, the author of this story, is part of a group of people from disciplines as diverse as medieval history, musical composition, neuroscience and poetry, who are in residence at the Wellcome Collection in London investigating what rest really means. 

The big question is: why is the idling brain so active? There are plenty of theories, but no agreement yet. Maybe different brain areas are simply practising working together. Perhaps the brain is staying active like an idling car, just in case it needs to act suddenly. But it’s possible that those mind wanderings and replays of our day play a vital role in helping us to consolidate our memories. We know that our dreams seem to play a part in sorting out our memories – now there is evidence that it happens during the day too (in rats, at least).  

We also know that when the mind is left to wander, it often focuses on the future. We start thinking about what we’re going to eat in the evening or where we’re going to go next week. All three of the chief areas of the brain involved in imagining the future are part of the default mode network. It is almost as though our brain is programmed to contemplate the future whenever it finds itself unoccupied.

Moshe Bar from Harvard Medical School thinks there might be a very good reason for that. He believes daydreaming essentially creates memories of events that haven’t happened. This gives us a strange set of “prior experiences” we can draw on to help us decide how to act if the daydreams ever do come to pass. For instance, many air travellers have wondered what it might be like to crash. Bar’s idea is that if the plane did actually crash, the memories of all those daydreams from previous flights would come into play and help the passenger decide how to behave.

But the resting state is not easy to investigate. As some cognitive psychologists have pointed out, just because a person is lying in a scanner we can’t be sure that they are alone in their thoughts, introspecting. They could be thinking about the sounds of the scanner and what’s happening around them. For this reason there are still plenty of unanswered questions about mind wandering. For instance, are the daydreams we experience when we’re trying – and failing – to focus on our work different from the ones we have when we’re deliberately trying to switch off?

Unique idleness

Progress is being made, though. A study published earlier this year hinted that we might all experience the resting state in a slightly different same way. Researchers conducted a detailed brain scan study of five people who had been trained to recount their mind wanderings in detail every time they heard a computer beep. The researchers found considerable differences between each person’s daydreaming thoughts and experiences.

In September researchers at the University of Oxford used scans from the Human Connectome Project of 460 people’s brains in a resting state to explore which parts of the brain communicate with each other when we are at rest. Again, the results hinted at personal differences in the resting state – this time linked to life skills and experiences. The strength of the connections between different parts of the brain varies with the strength of a person’s memory, their years of education and their physical endurance. It is as though parts of the brain remain connected when our mind wanders just in case we need them to do something.

The discovery of the resting state also has the potential to change the way we each feel about our brains

Scientifically, the discovery that the brain is never truly at rest could help make sense of a longstanding mystery: why does the brain uses 20% of body’s energy when the activities we know it performs should need only need about 5%? Marcus Raichle has labelled the missing 15% the brain’s “dark energy” – resting state activity might account for some of this discrepancy.

The discovery of the resting state also has the potential to change the way we each feel about our brains. We know how hard it is to empty our minds. We know how our minds have a frustrating tendency to wander even when we don’t want them to. But the emerging picture suggests these quirks might actually be beneficial – even if they do prevent us from finishing a task in time to meet a deadline. In other words, perhaps it’s time to celebrate the virtues of an idle mind. 

Follow us on Facebook, TwitterGoogle+LinkedIn and Instagram.