I started swimming for the reasons you'd expect from a 38-year-old father of two: cranky knees, balky back, the creep of middle-age spread. After years of broken promises, I resolved to give the pool a shot and to stick with it until it took. It was uncomfortable at first, but I kept going. After a month or so I was an embarrassingly full convert, donning cap and earplugs and ditching my old shorts for a (non-bikini) Speedo.

That was two years ago, and I did eventually get what I hoped for: I lost weight, my shoulders broadened, and my clothes all fit better. But I found myself enjoying the mental benefits of the swim above and beyond the physical ones. The sensation I crave begins to hit about halfway through, when the shock of the cold water and the early hour have worn off and the steady churn of hands and feet lulls my hyperactive mind into a repose I don't find any other way. Laps blur, and the demands of the day that had seemed so tiresome and daunting become, by the time I step out of the pool, so manageable and simple. It's like flipping a switch, and it works every time.

The science isn't revolutionary—exercise can reconfigure neural pathways to help fight stress, and aerobic activities in particular (swimming, running, cycling) can improve cognitive function and make us healthy in ways you can't measure with calipers.

These days, I don't like to make important decisions unless I've been to the pool that morning. Of my two possible selves, the one who goes for a swim and the one who doesn't, I no longer trust the one who doesn't. He wallows, makes excuses, runs needlessly afoul of the wife. Getting into slightly better shape has been nice, for sure, but it's the détente that a swim reliably (miraculously) creates between me and my own mind, my own life, that brings me back to the pool. It's the only New Year's resolution I've ever kept.