The intoxicating smell of Ambre Solaire is a distant memory and overhead the skies are darkening. Gone are the Orthodox Jews, with their Borsalino hats, long tresses and black breeches, and the legions of naked, sunbathing gay guys. Together these improbable bedfellows create the most incongruous changing room in London, at Hampstead Heath Men's Pond, during July and August. But now only a few dedicated swimmers remain to resist the remorseless advance of the seasons. The evenings may still be balmy, but the longer nights are pushing the water temperature inexorably down, risking dipping perilously below 50 degrees Fahrenheit. Even the swans are assuming a mournful air. Winter is the cruellest time for open-water swimmers, when pleasure gives way to pain.
I glance nervously at the blackboard by the wall. In the haze of rubbed out chalk I can just make out: "Today's temperature 12C, 53F". The atmosphere is Dunkirkian among the old campaigners, some of whom are in their eighties. Wisdom is conferred on those who have swum right through the winter. "It's the extremities that suffer most," one of them tells me.
Having pulled on a pair of shorts, I stride out towards the long jetty, like a condemned man seeking a dignified last walk to the scaffold. At the sight of the shimmering water my heart lifts. There are few more evocative sights than the Men's Pond, encircled by trees, bathed in the luminance of an November dusk. My pace shortens as the jetty takes me further and further into the middle of the apparently glacial pond. I reach the end. Whatever you do, you don't think about the water. The feet push off and I feel the rushing of air. Then everything happens at once. It's like that film where Woody Allen plays a sperm getting ready to be blasted off into uterine eternity: this is the moment that men in orange boiler suits are running around inside me to a cacophony of sirens, desperately trying to regain control. I resurface with a cry and fight for breath. After a few seconds I feel a burning across my chest, as if a frozen metal plate has been pressed to my torso. I think of Shadrach, Meshach and Abednego being thrown into Nebuchadnezzar's furnace and hope that I too will be saved.
The freezing pond seems huge. I'll head to the perimeter rope and then back - any further is too terrifying to contemplate. But after ten seconds a miracle occurs. Somehow all those men in hard hats pulling emergency levers inside me have got on top of the situation. I am able to stop swimming and savour the delicious cold. My mind turns to the great American swimmer Lynne Cox, who swam across the Bering Strait without a wetsuit at the height of the Cold War. I feel the blood rushing from my extremities to the internal organs and sense my brain go on to standby. Suddenly the leaves on the surface become icebergs, the swan on the far side is a Russian warship guiding me home, and the jetty is the Siberian shore full of waving Inuits and Soviet apparatchiks. I feel the cold nibbling at my legs and arms but it only drives me on faster. By the time I turn to complete the final approach to the jetty, I slow down to enjoy the numbness, which by now is pervasive. When I get out there's no KGB general to welcome me ashore, but I feel warm. The condemned man who has defied death receives a look or perhaps a nod of recognition from the others in the changing room. There are no heroes, only a camaraderie built on stoicism. And before anyone gets the wrong idea, this isn't a uniquely male phenomenon. I'm reliably informed that the same resilience is exhibited half a mile away at the Ladies' Pond.
A couple of years ago, the City of London authority, supported by the Royal Society for the Prevention of Accidents, tried to stop people swimming here on winter mornings. The swimmers brought a judicial review and triumphed against the risk-averse killjoys. Since then, the City, which assumed responsibility for the Heath after the Greater London Council was abolished, has installed a ticket machine charging £2 per swim that the rank and file have scrupulously ignored. They're quite right. Buying a ticket to swim in a pond is like paying to climb a mountain, walk through the woods or lie on a beach - the patenting of our natural highs.
For when you emerge dripping and glowing from the Men's Pond you are not only alive, you have slaughtered your woolly mammoth. In a flash, our atomised existence of email, mobile, iPod and BlackBerry is buried under an avalanche of endorphins.
The scientists seem to think it does us good, too. Later, online, I find a Dr Peter Amschel who has "proven conclusively that straddling a surfboard in very cold water causes the gonadal tissue to contract and shrivel, thereby stimulating and increasing the production of testosterone." The late Roger Deakin, doyen of open-water swimming, described in his book Waterlog how a study measuring the effect of daily cold baths resulted in weight loss, an improved immune system and increased testosterone levels in men and oestrogen in women. With wicked irony, Deakin noted that far from controlling the adolescent male libido, all those cold showers at boarding school must have sent young men wild with lust.
Somehow at the Men's Pond we manage to keep our appetites in check. A cheery grey-haired man who has swum right through, says the first winter is the easiest "as you don't know what to expect". But why does he do it? "It goes back to what Oscar Wilde said - the only cure for mental torment is physical pain," he laughs. One of the lifeguards comes through to lock up. "It's getting colder by the day, isn't it?" he says with relish. At first I have him down as a sadist. Then he mentions that the lifeguards have to go in three times a week during the winter for "habituation". You can get rather addicted to the buzz, he says.
Gonadal contraction is the answer, then, to winter blues. Just don't go telling the Royal Society for the Prevention of Accidents.