Numerous studies have linked cold water bathing to a reduction in the hormone cortisol.

In an article for Psychology Today, athlete Christopher Bergland labels cortisol “public enemy no. 1”.

While it performs lots of important functions, an undue increase in this chemical is related to a lot of different problems, including depression and decreased life expectancy.

“Once the alarm to release cortisol has sounded, your body becomes mobilized and ready for action—but there has to be a physical release of fight or flight,” writes Bergland. “Otherwise, cortisol levels build up in the blood which wreaks havoc on your mind and body.”

“Swimming serves… to sop us excess fight-or-flight stress hormones, converting free-floating angst into muscle relaxation,” writes swimmer Jim Thornton, in an article quoted by Psych Central. “It can even promote so-called ‘hippocampal neurogenesis’ – the growth of new brain cells in a part of the brain that atrophies under chronic stress.”

“It's about feeling alive”: the mental health benefits of wild swimming