Meditation: It’s celebrated as a therapeutic tool to help ease stress, anxiety, depression, addiction and chronic pain. It’s come into vogue as a way to enhance human performance, finding its way into classrooms, businesses, locker rooms and smartphones through apps such as Headspace and Calm. Various forms of meditation are now routinely offered to veterans with post-traumatic stress disorder.

In particular, mindfulness meditation, which focuses one’s attention on the present moment, is wildly popular and has ballooned into a billion-dollar business, according to the market research firm IBISWorld.

But for all its popularity, researchers don’t know exactly what the mindfulness version of meditation — or any other kind of meditation — does to the brain, how it influences health and to what extent it helps physical and mental challenges. People have practiced meditation for thousands of years, but psychologists and neuroscientists have studied its effects on humans for only a few decades. And many of those studies have used only small numbers of subjects, lacked follow-up and generally been less scientifically rigorous than other medical research.

“There is a common misperception in public and government domains that compelling clinical evidence exists for the broad and strong efficacy of mindfulness as a therapeutic intervention,” wrote a group of 15 scholars in “Mind the Hype, ” an article that appeared in January in the journal Perspectives on Psychological Science. Yet the reality is that mindfulness-based therapies have shown “a mixture of only moderate, low or no efficacy, depending on the disorder being treated,” the scholars added, citing a 2014 meta-analysis commissioned by the Agency for Healthcare Research and Quality.

Much more research is needed before scientists can say what mental and physical disorders, and which individuals, can be effectively treated with mindfulness meditation, they conclude.

Still, some studies do suggest that meditation that promotes mindfulness can help people relax, manage chronic stress and even reduce reliance on pain medication. Psychologist David Creswell, who directs the Health and Human Performance Laboratory at Carnegie Mellon University in Pittsburgh, thinks that sitting still and focusing one’s attention on the mere act of breathing, the core of many mindfulness meditation practices, can help.

Creswell first became interested in mindfulness meditation when he took courses on psychology and Buddhism in high school. Later, he began studying meditation in connection with reducing stress and improving overall health.

“As a scientist, I’m never convinced. I’ve been trained to be skeptical,” he says. “Nonetheless, I do think that there were a number of experiences I had while on meditation retreats that really struck me as very foundational.”

In a 2017 review of the scientific studies involving mindfulness, Creswell cites, for example, a 2003 study showing a correlation between practicing mindfulness and a number of indicators of well-being, such as self-esteem and life satisfaction

Some of the most impressive studies to date involve a treatment called mindfulness-based cognitive therapy, which combines meditation with psychotherapy to help patients deal with thoughts that lead to depression. Randomized controlled trials have shown that the approach significantly reduces the risk of depression relapse in individuals who have previously had three or more major depressive episodes.

Research bears out that people spend most or all of their day being anything but mindful. They skip from one thought to another. They daydream. They ruminate about the past. They worry about the future. They self-analyze and self-criticize.

In a 2010 study, Harvard researchers asked 2,250 adults about their thoughts and actions at random moments throughout their day via an iPhone app. People’s minds wandered 47 percent of the time, and mind-wandering often triggered unhappiness, the scientists reported.

From breathing to the lab

So alongside clinical work, neuroscientists now want to know how, if at all, mindfulness meditation might change what actually happens inside the brain. Does it make certain regions more active than others, or more robustly connect one region to another? Does it result in new neurons, actually changing brain structure? Some studies suggest the answer is yes.

Neuroscientists have studied the physical effects of meditation using functional magnetic resonance imaging (fMRI) and other techniques for about 20 years. During this period, there has also been growing appreciation that the human brain can change throughout adulthood, even into old age — forming new connections and neurons when someone learns a new skill, challenges themselves mentally or even just exercises.

The emerging view of a brain that can be continually shaped through experience, dubbed neuroplasticity, replaced the long-held idea that after the first few decades of life, the brain’s physiological trajectory was basically one of decline. A number of brain studies suggest that mindfulness meditation may spark neuroplastic renovations in the brain’s function and structure.

Looking under the hood with fMRI, scientists have found that mindfulness meditation may spark renovations in the brain’s function and structure. It appears to activate a network of brain regions that includes the insula (associated with compassion, empathy and self-awareness), the putamen (learning) and portions of the anterior cingulate cortex (regulating blood pressure, heart rate and other autonomic functions) and the prefrontal cortex (the hub of higher-order thinking skills such as planning, decision-making and moderating social behavior).

Some studies suggest that mindfulness meditation may increase gray matter density in the hippocampus, a brain region essential to memory. But what that means for real life is unclear. “We need to understand the benefits that the changes in the brain have on behavior and well-being,” says Britta Hölzel of the Technical University of Munich, co-author of a study that found evidence of an increase in gray matter density. “ ‘Changing the brain’ sounds very impressive, but we don’t understand what it actually means.”

It’s uncertain, too, whether such changes in brain activity can be sustained when the individual is not actively meditating, and if so, how much people would need to meditate for that to happen.

“Most of the data has only looked at changes over the course of two months of [meditation] practice. . . . Most people feel that [meditation] continues to change and get deeper with extended practice. So we need to conduct studies that follow people for much longer time points,” said one of Hölzel’s co-authors, Sara Lazar, of Massachusetts General Hospital.

A buffer against stress

Based on their own work, Creswell and his colleagues have proposed that mindfulness acts as a buffer against stress. It does this by increasing activity in regions of the prefrontal cortex that are important for “top-down stress regulation” while reducing activity and functional connectivity in regions associated with the brain’s fight-or-flight stress response — in particular, the amygdala.

Two studies by Creswell and his colleagues on small groups of unemployed adults experiencing stress show some initial findings that seem to support their view. Both examined the effects of three days of intensive mindfulness meditation training. One report found reduced functional connectivity between the right amygdala and a brain region that plays a role in modulating emotions.

Another reported increased connectivity between regions engaged when the brain is at rest and parts of the prefrontal cortex involved in regulating stress. That study also found that mindfulness meditation led to reduced levels of interleukin-6, a biomarker in the blood for systemic inflammation that’s elevated in high-stress populations.

PTSD, anxiety and pain

Similarly promising results were presented in a 2016 study of 23 combat veterans of Afghanistan and Iraq with PTSD. Brain scans before and after 16 weeks of mindfulness-based group therapy revealed increased connectivity between a network in the brain that allows people to control their attention and other parts of the brain involved in rumination and spontaneous thought. The level of connectivity was significantly correlated with a reported improvement in PTSD symptoms, including emotional avoidance and hyperarousal.

This particular connectivity has been seen in healthy people, as well as in people who have meditated for long periods, says study co-author Anthony King, a clinical psychologist at the University of Michigan.

“What’s important about our study . . . is that people with PTSD can also have this change in brain connectivity patterns when they do mindfulness practice,” King says. The more this connectivity increases as a result of mindfulness training, “the more their symptoms improve,” he adds.

Studies of other conditions suggest similar improvements — although many involve small numbers of subjects and other limitations that make them far from conclusive — for symptoms of general anxiety disorder and the perception of pain.

Mindfulness is an elusive quality to study. It’s an internally generated experience, not a drug that scientists can give to a patient. What’s more, there is no universally accepted definition of mindfulness or agreement among researchers on the details of what it entails, the authors of the “Mind the Hype” article note.

In the context of PTSD, King says it’s likely that mindfulness meditation will continue to supplement more-conventional psychiatric treatments. “I would never recommend for people to go to a mindfulness class at the YMCA or the local health center and think that that’s going to be the same as psychotherapy, because it is not. It really is not,” he says.

Still, people who use mindfulness meditation to ease daily stress say they’re convinced the practice improves their lives. One day, scientists hope to link that experience to what’s physically happening in the meditating mind.

This article appeared in longer form in Knowable Magazine.