All animals, Scott Carney tells us, seek comfort. But human beings are a bit different. We don’t need to spend much time actively seeking it. He’s right: it’s all around us — in your nice warm house, your air-conditioned car, your shoes, your bed, the temperate shopping mall you visit. Here in the affluent west, we eat comfort food in comfortable chairs, and then we recline on cushions, tweaking our dimmer switches and thermostats and adjusting the brightness on our screens.

Good for us, you might think. We can ‘control and fine-tune our environment so thoroughly that many of us can live in what amounts to a perpetual state of homeostasis’. That’s a scientific way of saying we’ve designed the world around us so we can feel good all the time.

But there’s a problem — ‘a hidden dark side’, as Carney puts it. ‘Effortless comfort has made us fat, lazy and increasingly in ill health.’ In other words, if you spend all your time trying to feel good, after a while you’ll start to feel bad. If you avoid pain for too long, you’ll end up in terrible pain — the pain of diabetes, obesity, autoimmune diseases, gout, piles, stiff necks and backs, bad hearts and weak lungs.

So what’s the answer? Let’s see: if seeking comfort ends up making us feel uncomfortable, what happens when we actively seek discomfort? As you might guess, it makes you feel great. Of course, it must be the correct type of discomfort.

Anyway, here’s what Carney does. He jumps into freezing water. He sits, shirtless, in the snow on the banks of a river in Poland. He climbs Mount Kilimanjaro in what might be record time (two days), stripped to the waist. He swims under water. He learns to hold his breath for ages. He trains with the toughest, most extreme athletes in the world — the surfer Laird Hamilton and the ‘high-intensity’ advocate Brian Mackenzie. Then there’s the Dutch guru Wim Hof, who is quite extraordinary.

Before I describe Hof, let me say that I believe he’s right — I always knew that jumping into freezing water makes you feel brilliant afterwards, but now I know why. Hof is a skinny chap with a scrubby beard. When he was 20, he jumped into an icy canal in Amsterdam. ‘The feeling wasn’t of cold; it was something like tremendous good,’ he says. That’s partly because the cold constricts your blood vessels, giving them a workout. It does lots of other things, too — you experience a rush of endorphins and a clarity of mind.

But there’s more. If you get cold regularly enough, it seems, good things happen to your body. You get better at controlling your temperature. The workouts strengthen your blood vessels. Also, Hof believes, you will develop ‘brown fat’, a substance that helps you to burn ordinary white fat as fuel, and ‘anti-inflammatory molecules’ in your blood. So: jumping into freezing water, and jogging in cold weather with your shirt off will help you in the long run.

And it feels good. We all know that. Getting into an ice bath, Carney says: ‘Endorphins buzz around my brain.’ Getting out, he says: ‘I feel almost perfect.’