I was at my wits’ end when I finally lowered myself into the River Wensum, in Norfolk, at the tail end of autumn 2019. My quest to find a cure for my severe anxiety and decades’ worth of weekly – sometimes daily – panic attacks was going badly. I’d exhausted conventional treatments, so I decided to try something different. An anaesthetist, Mark Harper, had told me about research he had co-authored into cold outdoor swimming as a treatment for depression.

In as little as six three-minute swims on consecutive days, in water of 10-14C (50-57.2F), I could transform the way my body responded to stress – or so the theory went. Immersion in very cold water induces something called the “cold shock response”: a gasp reflex, constriction of blood vessels close to the surface of the skin as your body tries to conserve heat, raised heart rate and hyperventilation. However, repeated exposures diminish the intensity of this response. You adapt.

“60% of your stress response as measured by blood pressure and heart rate changes,” Mark told me, “and 50% of that’s still there 14 months later.” The theory is that, as your body quietens the great hormonal cascade triggered by these short, voluntary exposures, your stress response is diminishing across the board – via something called “cross adaptation”. Then, when you’re feeling flustered in the wrong lane round a three-lane roundabout, your heart doesn’t pound as fast, you don’t pump out so much adrenaline and cortisol, and you’re better able to stay rational.

In Britain, wild swimming is like sex. Sometimes it’s a bit crap, but doing it matters less than cultivating a reputation as someone who does.

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It doesn’t start until your heart goes under. Then there’s the shock, the primal awareness that you’re a guest in an environment that cannot support you. After a minute of my first swim, my feet felt as if I’d crushed them with a breeze block. This was not some glorious communion with Gaia. I was an idiot in a very cold river.

When I got out, the towel felt like wire wool across my skin. My fingers and toes throbbed. I felt sick.

But it got better. After hesitating the next day, I scolded myself: come on, Tim, you’re briefly entering water of slightly suboptimal temperature, not going over the top at Passchendaele. The air temperature dropped to -2C – yet by then, bizarrely, I loved it. It was my favourite moment of the year.

And, for what it’s worth – I’ve never had a panic attack since.