Scientists have developed a new framework of how our mind wanders even when we are doing nothing, an advance that may help better understand mental illnesses such as depression, anxiety and attention deficit hyperactivity disorder (ADHD).

A new review of mind-wandering research, led by University of British Columbia in Canada, proposes a new framework for understanding how thoughts flow, even at rest.

"Mind-wandering is typically characterised as thoughts that stray from what you're doing," said Kalina Christoff, professor at UBC.

"Sometimes the mind moves freely from one idea to another, but at other times it keeps coming back to the same idea, drawn by some worry or emotion," said Christoff.

"Understanding what makes thought free and what makes it constrained is crucial because it can help us understand how thoughts move in the minds of those diagnosed with mental illness," she said.

In the review, researchers propose that thoughts flow freely when the mind is in its default state - mind-wandering.

Yet two types of constraints - one automatic and the other deliberate - can curtail the spontaneous movement of thoughts.

Reviewing more than 200 journals, researchers gave an account of how the flow of thoughts is grounded in the interaction between different brain networks - a framework that promises to guide future research in neuroscience.

This perspective on mind-wandering may help psychologists gain a more in-depth understanding of mental illnesses, said Zachary Irving, postdoctoral scholar at the University of California, Berkeley.

"Everyone's mind has a natural ebb and flow of thought, but our framework reconceptualises disorders like ADHD, depression and anxiety as extensions of that normal variation in thinking," said Irving.

"This framework suggests, in a sense, that we all have someone with anxiety and ADHD in our minds. The anxious mind helps us focus on what's personally important; the ADHD mind allows us to think freely and creatively," he said.

Within this framework, spontaneous thought processes - including mind-wandering, but also creative thinking and dreaming - arise when thoughts are relatively free from deliberate and automatic constraints. Mind-wandering is not far from creative thinking, researchers said.

"We propose that mind-wandering isn't an odd quirk of the mind," said Christoff.

"Rather, it's something that the mind does when it enters into a spontaneous mode. Without this spontaneous mode, we couldn't do things like dream or think creatively," he said.

The study was published in the journal Nature Reviews Neuroscience.