We may earn commission from links on this page, but we only recommend products we back?

Scientists, philosophers, sociologists, and many others have attempted to quantify this somewhat unquantifiable feeling. We all have heard that money doesn't buy happiness, but tell that to a single mother struggling to feed her children, and she might disagree with you. Certainly there is a chemical response involved with happiness, molecules like dopamine and serotonin stimulating tiny receptors in our brains, but provided that we have enough money to cover our basic needs and are chemically balanced, where and when do we find that we are happiest?

According to Lance P. Hickey, Ph.D., in the Huffington Post, many of us have reported feeling happiest when we are totally engaged in a challenging exercise or working on a difficult project. "These are moments in which your mind becomes entirely absorbed in the activity that you 'forget yourself' and begin to act effortlessly, with a heightened sense of awareness of the here and now (athletes often describe this as 'being in the zone')."

Perhaps a better term for this type of experience has come from the Hungarian psychologist Mihaly Csikszentmihalyi, who calls this condition "flow." I know first-hand what they are describing, as I have felt it often before. Perhaps never quite so distinctly as Sunday night in my semi-final of the men's 800 at the 2013 World Championships. In the final 13 seconds of that race I was flowing my brains out.

As I stretched in the warm, humid evening air before the race, I had an idea that the race was going to go well. Sometimes a warm-up is just so smooth and in sync that you can tell your body is ready for anything. As I stood on the starting line waiting for the gun to go off, my heart was racing, not with nervous energy, but with pure excitement. I needed to finish top two in the field of eight to automatically advance to the finals, and I was certain that my mind and body were up to the task.

The first lap unfolded as I expected it would. Moving down the backstretch of the second lap I positioned myself in second place, on the outside shoulder of the leader, Ayanleh Souleiman of Djibouti. We pushed each other hard through the final turn and as we came to the homestretch, with 100 meters to go, I knew that I was sure to advance. My position in the race was perfect and more importantly, I had spent very little energy to get into that position. Most importantly, my legs did not feel sluggish or leaden with lactic acid, but fresh and explosive. I took a quick peek to my right to make sure there were no hard-charging challengers and then began to embrace the moment, to truly flow.

In the 13 seconds that it took me to close that final 100 meters I allowed myself to really enjoy the moment. Instead of the usual race thoughts like, "How are my legs? Am I in the right position? Is the pace too fast? Too slow?" I was able to quiet my mind and just glide. The only thought I remember having was, "I am so lucky to be here, I hope my life is filled with many more moments just like this." Granted, 13 seconds is a blink of an eye in a lifetime of emotions, but I can assure you I have felt this way many times before while running.

I think this is maybe why a lot of us choose to run. In a chaotic world of traffic, iPhones, meetings, deadlines, etc., is there any place you feel flow stronger than when you are running? Fly fishing comes in as a close second for me, but nothing more effectively quiets my overactive brain or makes me feel more one with the world than the steady, rhythmic single purposefulness of distance running.