When's the last time you did something that was so engaging, so much fun, you lost track of time?
For some, it's belting out a karaoke tune. For others, it's exploring a new city or playing kickball.
That's what mortgage banker Sue Hamilton does for fun.
"It brings you back to a little bit of your childhood," the 41-year-old said.
Hamilton plays every week with a team at Atlanta's Piedmont Park.
"We all remember playing kickball from PE when we were in grade school. You can get hit by the ball without getting hurt, which I think is probably a good thing for most of us," said Hamilton, whose husband is also on a kickball team.
"I think most people go to work, and maybe there's not as much silliness or playfulness in their day ... and you do that with others, which is a plus for me -- that social component," she said.
'Play is essential to our health'
So, what exactly is play?
In addition to being the "action of a game," play is defined as "recreational activity; especially: the spontaneous activity of children," as well as "the absence of serious or harmful intent" and "a move or series of moves calculated to arouse friendly feelings."
"It's such a big thing that people want to hold it down to make it something small. It's not small. It's so important," said Dr. Bowen White, physician and founding member of the National Institute for Play, a nonprofit that supports research into the power of play.
"Play is so deeply ingrained in terms of our own evolutionary drive to survive, he said. "Play helps us connect with other people because we are open in a way that allows them to feel, maybe, this is a safe person to be with and maybe even fun to be around."
It's much more than a game, White said. And for adults, it's not necessarily just intimate-partner time.
It's a way of life, he said, that allows you to let your guard down, not be so serious and, as a result, connect better with others.
"We all come into the world knowing how to play," White said, adding that as adults, we shouldn't feel like we have to grow out of it.
"Play is essential to our health," he said. "If you want to have a fun life, you can't have a fun life without play."
Play should feel good, he said, adding that it has the opposite effect on the body as stress. Play often leads to laughter, which has been linked to decreased stress and inflammation and may improve vascular health.
"Your blood pressure goes down," he said. "You release dopamine."
People who play at work solve problems creatively
Play also makes you more productive, White said.
"What you get from the culture is 'work's important,' 'more is better,' " said White, who consults with companies on creating healthy work environments.
Adults, instead, should "figure out how to have more fun doing your work," he said.
People tend to perform better professionally when they're in a state of play, White said, noting that "playing" with problems at work sparks creativity.
Many companies are well aware of the power of fun to foster innovation. Google helped blaze the trail years ago when it set up play stations with ping pong, billiards and foosball tables all over its main campus. The company offered free snacks and exercise classes, and it let hundreds of software engineers design their own desks or work stations.
Google encouraged employees to play and collaborate, which it said helped with team-building and cooperation.
Working with professional sports teams, White has taught athletes to "play easy," rather than "playing hard," by easing up on a grip or holding their bodies more loosely.
"The best performances occur when they're playing easy," White said.
It's a metaphor, he said, for anyone trying to accomplish a goal: Loosen up a little, have fun, and you'll see progress.
"If you can figure out how to do that, you're going to be a lot happier and be more fun to work with," White said. "You'll be happier parents, a better spouse, a better partner."
The science behind play
The evolutionary importance of play can be demonstrated in the brains and behaviors of rats and primates, said Dr. Sergio Pellis, neuroscientist at the University of Lethbridge in Alberta, Canada.
In his experiments, he denied rats the opportunity to engage in play by fighting and wrestling.
Those rats developed deficiencies in their brains' pre-frontal cortex. This is the area responsible for executive functions, such as making judgment calls and emotional regulation.
"If you're an adult male rat put in another cage of a rat you don't know, the resident rat will see you as an intruder and beat you up," Pellis said.
If you're a normal rat, you'll find any place to hide -- a platform, perhaps -- and stay there, he said.
"If you're a play-deficient rat, you'll get beaten up and shortly thereafter move again and attract even more attention," he said. "You're not figuring out the appropriate thing to do in this situation."
Similar research with monkeys led to the same results, said Pellis, co-author of "The Playful Brain: Venturing to the Limits of Neuroscience."
While this type of experiment cannot be replicated in juvenile humans for ethical reasons, social science studies have shown kids who engage in more play end up with higher social skills a few years down the line, Pellis said, predicting more research in the next decade on adult human brains at play.
Kickball players make friends on the field
For 30-year-old kickball player Wesley Brown, an urban planner in Atlanta, the opportunity to take the field is truly a "social lubricant," a phrase White uses to describe play.
"I've made friends through kickball," Brown said. "There's no way that I would've ever met any of these folks before: very, very different backgrounds, very different professions.
"Through playing and having fun together, we organize different activities during the week," he said. "We have a baby shower that we planned. We have a wedding shower. We're a very close-knit team."
Hamilton, the banker, feels the same way.
"It's an easier way to create conversation because there is a definitive topic, whereas, maybe, meeting somebody somewhere else at a bar or something, it's not always easy to have a shared conversation," she said.
And then there's the fun of just letting go.
"I feel like I'm a kid most days, so, for me, this is just another aspect of that," Hamilton said. "I like to laugh a lot. I like to run around and be silly."
Hamilton loves "being able to have that recess time that we did when we were younger and you can run around the field and be free of your daily responsibilities for at least an hour," she said. "You don't have to worry about work. You're not thinking about whatever personal things might be going on in your life."
For White, play is a way of being in the world -- being more "playful."
"The person at the grocery store that's in line, can you goof with them? Or are you just in a hurry: 'Oh, they've got 14 items'; you counted them, and it's 12 or less. 'I can't believe that!!' How does that feel being in that psychological space?"
Life, he said, is much easier with a playful approach.