Awhile back, I remember flipping through a magazine and happened across an ad depicting a triathlete racing with a rigid grimace and white-knuckled bullhorn death grip. It struck me as funny because not an hour earlier I had just written about triathlons, describing it as “recess,” “play,” and “ridiculous fun.”

This man did not look like he was having any fun, let alone the ridiculous variety. So, in keeping with the theme, I had a little fun on Facebook at his expense. But now I suspect I was a bit hasty in my judgment. It’s true he was probably not having the time of his life at that very moment. He was likely sailing at 120 percent of his lactate threshold power output and playing mental Whack-A-Mole with his ever-increasing negative thoughts. Not a particularly fun moment, no.

But he was also probably getting higher than a hippie at a RatDog reunion.

This ties in with my interview of John Ratey, MD, author of Spark: The Revolutionary New Science of Exercise and the Brain. He was gushing about the feel-good effects of exercise of all types, but particularly the tip-top high-intensity kind. The teeth-gritting, vein-popping effort the “not playing” man was doing unleashes a veritable flood of happytime hormones and neurotransmitters, including dopamine, serotonin, nitric oxide, and endocannabinoids (that chemical sound familiar?). He was also pumping out copious amounts of human growth hormone—a naturally occurring hormone that increases muscle mass and fat-burning, and as the name implies, helps keep our bodies young.

It’s such a potent performance enhancer that synthetic versions are banned by the International Olympic Committee and hunted by the anti-doping police. A study in the Journal of Sport Sciences reported that levels of human growth hormone skyrocket 530 percent after just 30 seconds of sprinting on a stationary bike. Heck, this guy was HGHing himself straight back to adolescence.

And maybe that’s the point. You can put on your game face, get intense, rage like your life depends on it, work your tail off, and be all business in your sport. Then, when you’re finished, you can slump over your bars, crack a giant cat-ate-the-canary grin, drink a beer, and feel like a kid, already anxious for the next time you can get out and play.