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And the number of the initiated is swelling. Quantified Self has more than 24,000 members worldwide, and a recent Pew Research Center’s Internet & American Life Project survey found that seven of 10 U.S. adults regularly track an aspect of their health or that of a loved one. Weight and diet are the most popular things to monitor, followed by blood pressure, blood sugar and sleep patterns. Many of these self-trackers store the information in their heads or write it down on a piece of paper, but 21 percent of those surveyed use a form of technology to record the data.

Derek DiLuzio, a 30-year-old lifestyle and professional-sports photographer in Asheville, N.C., and his girlfriend, Sarah Holland, began tracking their sleep via a smartphone app after Holland developed insomnia. The app, called Sleep Time, confirmed what they already knew: They woke up several times during the night thanks in part to their active dog and cat. But it also inspired them to research holistic solutions for Holland’s sleep woes. “We found valerian root,” says DiLuzio, referring to a natural supplement that has shown signs of improving sleep. “Had we not been vested in analyzing our sleep, we probably wouldn’t have taken the next step.”

Sure, there are the eccentrics who track toenail length and the number of times they chew each piece of food, but Dean, who once charted the efficacy of various ointments that doctors had prescribed to quell an itchy rash on his eyelid, says most individuals use the technology to solve a problem. “They’ve not had good luck in the traditional health-care field, so they’ve taken it upon themselves to get underneath the condition or situation,” he says.

That’s the story of 34-year-old David Goldstein, a cancer survivor who, in 2005, was hit by a bus, worsening the chronic pain, depression and weight gain caused by the cancer. After his son was born in 2011, he decided to take control of his life. “I know how to fix my clients’ problems by looking at the data, but I didn’t know the data for me,” says Goldstein, a research analyst in New York City.

Then he met Joshua Manley, a data-driven health coach. Manley customized the app Mymee to track Goldstein’s diet, weight, pain and stress levels, exercise, sleep patterns and frequency of medication. The two met once a week for three months to analyze the data, and they soon discovered that, on days Goldstein performed certain exercises and stuck to his predominantly vegan diet, he experienced significantly less pain. “I finally cracked the code for weight and pain,” says Goldstein, who hovers at his goal weight of 180. “There was a pattern of bringing those two things under control, but I couldn’t figure out what it was. Doctors all had guesses and suggestions, but no one was able to say definitively what would do it.”