July 13, 2024

I, Science

The science magazine of Imperial College

A winter pastime for the intrepid -- or for the merely foolhardy? A look into the physiological and mental effects of cold-water dipping.

by Molly Rains on 29/12/2022

Standing at the edge of deep, frigid water on a grey November morning, the reasons why you shouldn’t jump become much easier to recall than the reasons you should. This was the case for me a few Sundays back at Hampstead Heath. It was only 7:30, and the sky was still half-dark and heavy with the last dregs of an overnight rain. Before me, a dozen or so women of the Ladies’ Pond – “pondies,” they’re called – made unhurried laps through the dark water, chatting as they swam, their laughter suspended in glistening clouds over the surface. They looked joyful, and free, and I had come across town to join them, but even dipping my foot beneath the waves was enough to make my breath catch. So I hovered, clutching my towel around my shoulders, in limbo on the shore.

The human aversion to cold water is for our own good. Relative to other mammals, whose thick fur coats, strategically placed body fat, and adaptive metabolisms are designed to keep core temperature stable in conditions as extreme as arctic winters, humans are poorly equipped to handle low temperatures. For us, it only takes exposure to water below 21° to cause hypothermia.

To keep us from freezing, we’ve evolved a set of physiological warning bells that chime to signify dangerous conditions. Many extremely sensitive temperature-detecting neurons lie in the skin, sensing a chill when skin is cooled as little as one degree. The more unpleasant sensation known as cold pain – the stinging, prickling sensation of extreme cold – is brought on, for most people, by exposure to temperatures 12° or lower. Cold temperatures below this threshold, like those a winter swimmer experiences, trigger the sympathetic nervous system and the so-called “fight-or-flight” response.

For the unacclimatised, like me, this means that cold-water immersion is at best unpleasant and at worst dangerous. For the seasoned cold-water swimmer, however, the shock is something to look forward to. Talia Sperduto of New Hampshire, U.S.A., has taken a dip in the chilly Atlantic every morning for the past 76 days and counting. “It’s kind of an addictive feeling. I don’t know how to describe it any other way,” she told me on day 46. Her swim that morning was in water of only 9.4°, the air a brisk -1°. I am aghast. “It didn’t feel so bad,” she says, laughing.

That cold-water enthusiasts like Talia can endure near-freezing water with a smile is a testament to the human body’s adaptability. Through repeated cold exposure, winter swimmers’ bodies become uniquely cold-tolerant in a process known as cold acclimatisation. Researcher Dr. Susanna Søberg studied this effect in Scandinavia, where cold-water dips alternated with sauna sessions are a popular winter pastime. Her team found that, compared to non-practitioners, winter swimmers experienced a less severe stress response upon cold exposure. The swimmers also produced more body heat to compensate for the cooling effect of the water. Søberg traced this to increased activation of brown adipose tissue, or “brown fat,” which burns energy to keep the body warm.

On that Sunday morning at the Heath, unacclimatised and half-asleep, I stood shivering as my feet went numb. I’d come too far to turn back now, though – so, gritting my teeth, I slipped into the water. At first, I could neither breathe nor think. The cold stole the air from my lungs and the thoughts from my head and it was all I could do to keep paddling. But after a minute, my breathing slowed. My limbs were suddenly light and buoyant, and my head felt filled with air. Søberg attributes this to the temporary reduction in blood flow to the brain that accompanies cold immersion. Talia described it, too: “It’s such a refreshing feeling… like clearing out everything that you don’t want in your body and brain,” she said. “A fresh start.”

I have a long way to go before I’m a true cold-water swimmer – but as I climbed out of the pond into the strengthening sunlight, experiencing something like euphoria, I knew I was hooked. A woman towelling off beside me gazed over the water and began to laugh. “We must be mad,” she said, shaking her head, watching the knit caps gliding over the surface.

Mad? I’m not sure. Dedicated? Absolutely. Winter swimming requires commitment and courage. The devotion of its practitioners is a reminder of just how adaptable the human body is, and how positive, intentional habits have the power to effect fundamental changes in our bodies and minds. We aren’t static beings. We’re changing every minute, and with intention we can shape those changes to help us better endure discomfort, keep commitments, and act mindfully. Of course, this is all easier written than done – but if you want a friend along the way, you can find me at the Ladies’ Pond.