Extreme cold training that promises to reset the way your body works and how you sleep.

It looks, for all the world, like a shower. One of those corner arrangements. The type you get in new-builds in Cheltenham or Portland or anywhere else that aspires. Unlike the showers of Portland or Cheltenham, though, it glows moon-blue and crystals the size of snowflakes dance a paso doble on the floor, as if conducted by some unseen hand. But then it wouldn’t look like them, because this is a cryogenic chamber with an interior temperature of -87°C.

“In you go,” says the kindly voice of one of the therapists at London’s KXU as I climb inside, naked but for a headband, face mask, sneakers and shorts. “I’ll start the music when you get in.” And so she does and my week takes a singular turn.

The Chelsea-based fitness and wellness “boutique” is the spin-off of a nearby members health club called KX, which costs £16,000 to join and offers a dedicated wellness cryo recharge – including freezing, nutritional advice and exercise classes. At KXU, the classes are pay-as-you go, and is where Prince Harry got in shape for his wedding. No word if he went in the freezer, mind you.

Under the tutelage of the Wellness director at the club, Mr Gideon Remfry, I have signed up for a five-day programme. Each morning, I arrive at KXU, drink a cup of green tea, get changed, then for three minutes I go in the freezer while listening to Fleetwood Mac, and then, thawed out and revivified, I raise my temperature in an intensive 30-40 minute exercise class. After which I drink a green juice with kale, rocket, parsley and matcha. It was new experiences all round.

This type of intensive mind and body fitness programme – often yoked beneath the term “wellness” – would have seemed fanciful to the standard Nike-clad athlete not so long ago. It was the preserve of Messrs LeBron James and Cristiano Ronaldo, both of whom use it to hasten recovery time (more of which anon). But the fitness market has changed: today it rages. There are, as of last year, 9.7 million gym goers in the UK, that’s one in seven of the population – a whole five per cent more than the year before. The market is worth £4.7bn and to sustain, never mind expand, the fitness vanguard has to branch out. And as well they might, given that last year alone, 272 new fitness centres opened in the UK.

And so it is that I come to be spending my morning in a freezer. Before the cold splash, I meet Ms Kathryn Fielding, the community manager here, who has me remove my shoes and step onto a large device that looks like electronic scales. “Splay your fingers and hold still,” she says, as current travels from my feet to my fingertips. This machine records my lean muscle mass, hydration levels and fat mass (subcutaneous and that around my organs). I will do this again once the programme is over.

The five-day programme costs £790 and is not simply concerned with slimming down the waist and bulking up the abs, it is supposed to also help with sleep, brighten the skin and increase energy levels, while also allowing you to exercise with a greater frequency as the cold is said to help with post-gym recovery. Freezing yourself works, in essence, by shocking the body and increasing blood supply to your muscles. It is like inducing a fight or flight mechanism, so the endorphins pour in and the metabolism perks up.

It is a peculiar sensation, to stand there, repressing the desire to get out of what amounts to a high-tech chest freezer. The cold seems to wash over you, tickling the limbs, freezing the hairs of the legs. But it isn’t an unpleasant sensation. The unpleasant part is being shut in; even if the therapists stand just beyond the glass being jolly. And yet you are only inside for a short time, and when you do emerge, there is an unmistakable rush, and the muscles that ached so unswervingly an hour ago, are soothed.

I cannot say I divined any improvement in sleep, but I am pleased to announced two MR PORTER colleagues took the trouble to say, “You look well” and “Your skin looks nice”.

Even if KXU advertise its all-round wellness potential, many people will likely join the programme for that unassailable reason: vanity. You do lose weight. The exercise, the nutritional guidance, the juices, the deep-freeze, together they slice the fat like a knife through, well, lard. I reduced my BMI by 0.5, lessened fat mass by 1.4kg, increased skeletal muscle by 1.1kg and lost 1cm off my waist. Of this I was well pleased. So much so I went to Italy and ate lots of spaghetti carbonara. Perhaps I need another week in the freezer.