In late 1971, US Navy veteran Stephen Islas returned from Vietnam, but the war continued to rage in his head. “I came very close to committing suicide when I came home, I was that emotionally and mentally damaged,” Islas remembers. At his college campus in Los Angeles, a friend suggested he check out a meditation class. He was sceptical, but he found that before long “there were moments that started shifting, where I was happy. I would experience these glimpses of calmness.”

Forty-six years later, Islas says that he has never completely freed himself from his post-traumatic stress disorder (PTSD), which was formally diagnosed in 2000 at the Veterans Affairs (VA) West Los Angeles Medical Center. But he’s convinced that meditation has saved his life.

Various forms of meditation are now routinely offered to veterans with PTSD. It’s also touted as a therapeutic tool to help anyone suffering from conditions and disorders including stress, anxiety, depression, addiction and chronic pain. More broadly, meditation has come into vogue as a way to enhance human performance, finding its way into classrooms, businesses, sports locker rooms and people’s smartphones through Internet apps like Headspace and Calm.

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‘Mindfulness’ meditation, a type of meditation that focuses the mind on the present moment, is wildly popular. It has even become a billion-dollar business.

For all its popularity, however, it’s still unclear exactly what mindfulness meditation does to the human brain, how it influences health and to what extent it helps people suffering from physical and mental challenges. Meditation has been practiced for thousands of years, but psychologists and neuroscientists have studied it for only a few decades.