Coloring books aren't just for kids anymore — they're for adults too, even all grown up pop stars like Justin Bieber.

Bieber showed his pride by posting a completed page from an adult coloring book on Instagram Wednesday. The 22-year-old singer isn't alone in his appreciation for adult coloring books, as many are discovering the mental health benefits of an art form often forgotten after we leave the halls of elementary school.

Say what you will about the scantily clad, Tomahawk-weilding anime figure — whose hair Bieber colored grey and whose shirt he envisioned as light purple — but the practice of sitting down with a semi-blank canvas and completing an illustration can be therapeutic. Bieber celebrated the fact that he "actually sat down for 30 min and finished," adding "I never can sit still for that long so it's an accomplishment."

Adult coloring books are on the rise in the U.S. The New York Post called it the "hottest trend in publishing" and Fortune attributed a marginal rise in printed book sales in 2015 to adult coloring books. Indeed, an Amazon search for "adult coloring book" turns up nearly 25,000 hits today, revealing the explosive growth of this seemingly niche market. The reason behind the adult coloring book surge could be that people, like Bieber, are finding an effective therapy option between the bolded lines.

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The mental health benefits of adult coloring books.

"Coloring definitely has therapeutic potential to reduce anxiety, create focus or bring [about] more mindfulness," Marygrace Berberian, a certified art therapist and assistant professor at NYU's Graduate Art Therapy Program, told CNN. "My experience has been that those participants who are more guarded find a lot of tranquility in coloring an image. It feels safer and it creates containment around their process."

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A 2005 study found that coloring mandalas (frames with various geometric patterns inside), as opposed to simply doodling, effectively reduced anxiety in study participants, lending credibility to the concept of art as therapy and also demonstrating the psychological benefits of completing a creative task that doesn't require invention. Similarly, one of the fathers of modern psychology, Carl Jung, instructed his patients to color as a relaxation exercise meant to calm and center people.

"Because it’s a centering activity, the amygdala, which is the part of the brain that is involved with our fear response, actually gets a bit, a little bit of a rest, and it ultimately has a really calming effect over time," Dr. Ben Michaelis, a clinical psychologist, told Fox News.