In November 1782, the diarist Fanny Burney wrote the following about her 6am sea swim: "It was cold but pleasant. I have bathed so often as to lose my dread of the operation which now gives me nothing but animation and vigour."
Fast forward two hundred years or so, and our passion for an icy dip shows little sign of having dimmed. My Facebook feed is full of people in swimsuits, creeping into biting seas or clambering out of freezing pools, all grinning their heads off. Quite a few are wearing daft hats.
![Step in to perk up.](/web/20170223164422im_/http://www.theage.com.au/content/dam/images/g/u/e/5/k/m/image.related.articleLeadwide.620x349.gue4lg.png/1487794654629.jpg)
In Fanny's day, doctors thought cold water immersion was good for your health and the colder the better. Today, our lives are differently circumscribed in almost every way; Fanny would have worn more clothes in the water than we do fully dressed.
But on the subject of cold swimming, she and I are in total agreement.
![More people are taking the plunge.](/web/20170223164422im_/http://www.theage.com.au/content/dam/images/g/u/e/5/k/8/image.related.articleLeadwide.620x349.gue4lg.png/1487794654629.jpg)
I too have lost the "dread of the operation", and have that same animation and vigour. As do the growing numbers of people partaking in this most foolish of activities. Cold and outdoor swimming in general are on the rise across the UK. Membership of the Outdoor Swimming Society has rocketed from just 300 in 2006 to more than 25,000 in 2016 (there are many year-round ocean swim groups in Australia too).
Last weekend, 80 people took part in the Great Britain Ice Swimming Championship in Loch Lomond, swimming 1km in water between 0C and 5C. Organiser Pauline Barker told me that, such was the volume of applicants, they could have filled the event twice over.
Who are these people? My social media feeds show a mixture of ages, genders, shapes and sizes. The one thing we have in common: at some point, someone will probably have asked us only semi-jokingly, "are you mad?" My reply, in as lofty a tone as I can muster, is "actually if I were mad, cold water swimming might help".
Because it seems Fanny's doctors might have been right.
Most cold water swimmers are enthusiastic salesmen for its physical health benefits. We gleefully proclaim that we get fewer colds, which has now been borne out by research conducted by the Extreme Environments team at Portsmouth University.
"Cold swimmers have better resistance to colds than their partners," they concluded, although the question of why it brings improved immunity is part of their ongoing work. Cold water is also associated with inflammation relief - think footballers in a post-match ice bath - and one Finnish study found that winter swimming helped people with pain from rheumatism, fibromyalgia and asthma.
Dr Mark Harper, the Outdoor Swimming Society's expert, cites "better metabolism, especially with regard to the way the body handles sugar" as another benefit, and talks of how repeated immersion helps 'train' our fight-or-flight response - the icy temperatures being several degrees below the tropical climes of our evolutionary origin - meaning that we might become better at coping with stress, "a bit like the controlled exposure used for allergies", he says.
The Extreme Environments team is also looking for definitive evidence that cold water swimming could help our mental health. In the meantime, there is good theory. We know that connecting with nature helps urban dwellers to reduce anxiety; that natural light, even a feeble wintry one, boosts our levels of serotonin and vitamin D, both linked to depression.
Dr Harper cites cold water as improving mood and creating a feeling of, yes, exhilaration (a massive endorphin boost will do that).
Barker describes an "immediate rush of adrenalin, even as you get in, that leaves me smiling for the rest of the day".
The Finnish study reported that, "after four months, the swimmers felt themselves to be more energetic, active and brisk", concluding that "improvement of general wellbeing is thus a benefit induced by regular winter swimming."
But if the science is still cautious, the anecdotal evidence is near-unanimous. You only have to ask cold water swimmers if it's helped them physically or mentally to get an overwhelming "yes", on both counts.
Dr Harper calls it "anecdote based on theory", and it builds a convincing narrative. Out in the icy water, stories can resonate more fiercely than the science.
Alexandra Heminsley, author of Leap In: A Woman, Some Waves and the Will to Swim, says she feels that "cold water swimming helped me with the most difficult emotional challenge of my life: dealing with infertility. Just as my body seemed to be letting me down, the regular act of heading to the sea and convincing myself to withstand and enjoy the water was a regular solace. It taught me that I can endure more than I had let myself believe."
A woman I spoke to for my book Swell: A Waterbiography told me how she learnt to swim in Hampstead Ladies' Pond after the death of her partner "to cancel out the grief, to wash it away".
"One day I came here," she said, "and swam underwater. I felt like it was literally holding me in its arms". Water as a curative - just like those 18th-century doctors said.
Another, Natalee, told me how, having struggled with depression after losing her baby son, she joined the Chester Frosties outdoor swim group. She'd seen a BBC documentary on the subject and thought it might help. "Being close to nature makes me feel closer to him," she said. "Swimming has a very calming effect. It's exhilarating in a positive way."
Barbara Jennings is unequivocal that swimming in the cold waters of Tooting Bec Lido helped her deal with depression. "When you're getting into and swimming in cold water, it's very difficult to actually think about anything else," she said. "You are totally focused on the physical sensations, on the effort of will required to get yourself in and keep yourself in, on the effect that cold water is having on your body."
It is a powerful feeling; that your body works despite it all. The physical demands of struggling against the piercing cold can release you from what's going on inside your head. That sense of "being in the moment" was helpful for another grieving swimmer: "When all your senses are being fired there is little space for extraneous thought. I found that dedicating each swim to a particular memory helped me to move forwards a bit."
Friendship, or at least companionship, is another significant draw. "The camaraderie is a very big part of it," says Barker. Community has its own mental health benefits; isolation can be deadly.
But there is no magic here; just water. Perhaps, one day, science will completely unpick the physical benefits of cold swimming. Maybe it will remain elusive and anecdotal. The main thing is, should the urge take you, to take the plunge and join us in grinning your head off. Daft hat not essential.
The Telegraph, London