Coloring books aren’t just kids’ stuff anymore. Adult coloring book sales have exploded in the past year, as studies show Americans are turning to them to ease stress in a creative and relaxing way, according to a new analysis of the publishing industry.

Nielsen’s just-released U.S. Book Industry Year-End Review reports that 12 million adult coloring books were sold last year, compared to just 1 million the year before.

Many studies have found coloring books are making a comeback as a nostalgic stress-reducer for baby boomers, mirroring renewed interest in knitting, handicrafts, and other meditative activities that are gaining popularity with millions of Americans.

“In this hectic world where the only news we seem to get is bad news, coloring provides a safe, stress-free respite. A place where we can feel both calm and creative," says Kim Weiss, communications director of HCI Books, which has produced a line of adult coloring books called “Inkspirations,” that include positive message similar to those in the “Chicken Soup for the Soul” books, also produced by HCI.

“Scientists have studied coloring, and they've found that it quiets your mind, calms your thoughts, reduces stress, and allows you to simply be. Research shows that coloring can induce a kind of 'flow,' or active meditation, during which you lose your sense of time and your brain waves fall into a calming rhythm. As a result, worries fade away and creative blocks can become wellsprings of ideas.”

Florida artist and entrepreneur Diego Orlandini, who has just published “The Wynwood Coloring Book,” says the pastime is a great relaxation technique that harkens back to childhood for many baby boomers and other coloring fans.

His 64-page book (which sells for $25) was inspired by the street art of Wynwood, Miami, and features a full-color directory of the names of the participating artists. Among them: Alex Senna, Alexander Mijares, Diana Contreras, Jason Botkin, Jenny Perez, Jim Mahfood, Alice Mizrachi, Chris Riggs, GG, Indie 184, Patch Whisky, Pawn Price, Sheryo & The Yok, and Uncutt.

"Our mission is to give you the opportunity to engage your creative mind while following the steps of your favorite street artists, reimagining the colors of Wynwood’s famous walls," he says.

Orlandini says he started coloring and doodling years ago as an entertaining way to relax.

He evidently has plenty of company. The new Nielsen report indicates adult coloring books were the fastest-growing category of publishing sales last year.

“We’ve all heard the saying ‘Everything that’s old is new again.’ In the book realm, that statement couldn’t ring more true,” the Nielsen report’s editors noted. “Adult coloring books … had a breakout year.”

Other sales trends noted by the report:

• Conventional print book sales increased 3 percent last year over 2014.
• E-books’ share of the market dropped from 27 percent in 2014 to 24 percent last year.
• Readers of digital books are increasingly turning to their smartphones, with e-book consumption via smartphone growing from 7.6 percent in 2014 to 14.3 percent in 2015.
• Two categories of books showed stronger digital sales than print: Romance novels and thrillers.
• By category, non-fiction book sales were strong, with a 12 percent increase in children’s non-fiction and 7 percent growth in adult non-fiction. On the fiction front, the big gainers were science fiction (44 percent), classics (32 percent) and graphic novels (22 percent).

The findings are based on an analysis of 653 million book sales, according to Nielsen.