It's touted to relieve pain, lower stress and anxiety, and bolster cognitive performance, but does the practice of mindfulness physically change the brain — and if so, how do we know?
First, it's important to know that mindfulness programs can take many forms, from free mobile apps to highly structured, weeks-long guided sessions. So all these different habits are likely to produce different effects.
In neuroscience research circles, there are two major mindfulness regimens: mindfulness-based stress reduction and mindfulness-based cognitive therapy.
People undertaking the mindfulness-based stress reduction course receive eight weeks of intensive mindfulness training, which takes elements from practices such as meditation and yoga.
It's been around since the 1970s and, as its name suggests, it was created to alleviate anxiety and stress.
Mindfulness-based cognitive therapy, on the other hand, was primarily designed for those with depression. It weaves aspects of mindfulness — like meditation — together with a type of psychotherapy.
In the past decade, high-resolution brain imaging has let us look beneath the skull and find out what effects these mindfulness programs have on the way the brain works.
How mindfulness shapes the brain
It's still a burgeoning field, but a few different studies have suggested mindfulness interventions increase the volume of brain regions that help regulate emotion and attention.
One area that seems to get chunkier is called the anterior cingulate cortex, according to Neil Bailey, a neuroscientist at Monash University.
"The function of this region seems to be related to choosing between competing brain processes," he said.
"So if you've got a part of your brain that's saying 'eat the doughnut' and another part of your brain that's saying 'no, do your homework', the anterior cingulate cortex is the part that decides which to focus on."
If the anterior cingulate cortex focuses attention, it's the prefrontal cortex — responsible for complex cognition — that sustains that focus. It also seems to thicken up with a bit of mindfulness training.
The hippocampus and amygdala, which are primarily responsible for memory and emotional processing respectively, change as well — in the strength of their connections.
When the amygdala is activated, it can trigger the "fight or flight" response, the reflex that gets your heart pumping and your body ready to react to a threat.
It's thought that some people with anxiety disorders have a hyper-responsive amygdala, eliciting fight or flight even when there's no life-threatening danger.
After a bout of mindfulness training, the amygdala might still kick in, but messages feeding into it from the prefrontal cortex and hippocampus that give context to the situation — that it's not a life or death scenario — may be stronger.
"What research has shown is that there's less amygdala activity after a mindfulness intervention and that it's related to the downregulation by the other brain areas," Dr Bailey said.
Measurements aren't perfect, but are improving
Dr Bailey uses a technique called an electroencephalogram, or EEG, to look at how mindfulness changes the brain's function.
It's a cap of electrodes that can pick up electrical activity in the brain's outer layers.
The system has been used for many years and is very reliable, but relatively indirect.
To get right inside the brain and find out if its shape and size are changing, the best technique we have is magnetic resonance imaging or MRI.
These scans are the closest we can get to cracking open a person's skull and taking a ruler to their grey matter. It's the technique used in the studies, mentioned above, examining the cortex after mindfulness training.
An MRI machine records virtual slices through the brain (or any other squishy part of the body).
It was developed in the 1970s, but in recent years, MRI resolution has improved vastly thanks to more powerful electromagnets.
"The stronger the magnet, the better the image," said Rebecca Koncz, a neuropsychiatrist at the University of Sydney who is also completing a PhD in brain imaging at the University of New South Wales.
MRI these days commonly operates with a 1.5- or 3-tesla magnet — around the strength of the electromagnets that hoist cars in scrapyards.
Newer, more expensive MRI models use a 7-tesla magnet or higher.
As well as more precise size-and-shape measurements, these superpowered magnets are proving useful for taking snapshots of the brain in action: a sort of whole-brain activity map called functional MRI.
This "fMRI" tracks oxygen in the brain's blood supply, to see which parts are working harder. The idea here is that more activity needs more oxygen.
It's the type of evidence which revealed, for example, that mindfulness training can dampen the activity of the amygdala.
Still, even top-shelf MRI machines have their limitations.
They divvy up images into 3-D pixels called "voxels". Depending on the thickness of the slice, one voxel is usually around 1 cubic millimetre.
A morsel of brain tissue that size can contain tens of thousands of cells, so slight density or volume changes might go unnoticed.
And the difficult, indirect analysis required to quantify brain activity from fMRI data has led to criticisms about whether the results are meaningful.
There's plenty left to discover
The neuroscience of mindfulness has drawn on all of these techniques — but is still in its infancy.
There are plenty of questions left to be answered, such as: Does the quality of mindfulness practice matter? Is there a dose-dependent effect (i.e. more mindfulness, more change)? And are there long-term effects on the brain?
Some evidence shows that compared to the wider population, long-time meditators have more brain volume in, for instance, the prefrontal cortex.
But there are no longitudinal studies as yet that have followed novice mindfulness practitioners to see if they maintained their newly gained brain bulk.
Even though the benefits of mindfulness can sometimes be overstated, there is likely to be a good reason it's been around for so long, Dr Koncz said.
"Mindfulness and meditation have been practised for thousands of years. Perhaps science is just taking a bit of time to catch up."