The Finnish tradition of a sauna followed by a plunge into icy water or a roll in the snow is spreading around the world. What does science say about the claimed health benefits?

Sometimes I wonder why I do these things to my body. I'd just been for a mid-winter run in a frozen park in the Finnish city of Oulu. The air temperature was -15C (5F). At 170km (106m) south of the Arctic Circle, this is the world's northernmost Parkrun.

Less than an hour later, I ducked into a sauna – a dimly-lit wooden room where intense heat was radiating from a massive wood-burning stove. The thermometer read 85C (185F).

As I sat there, sweating like crazy, a fellow sauna-goer threw a ladleful of water onto the stove's hot stones to make "löyly" – Finnish for a sudden burst of steam that surges through the air. It's a powerful sensation, almost shocking. The heat reached deep into my skin and muscles, melting away any tension.

But that wasn't enough for me. Stepping outside into the cold air, steam rose from my body, and I clambered down an ice-encrusted ladder into the excruciatingly cold water of the Oulu River, which had a temperature of around 1C (34F). I felt piercing pain, like a million needles striking my skin.

But I didn't jump out. Instead, I counted slowly up to 12. And after a few seconds the pain seemd to disappear – I actually felt okay with it. It was almost like an icy hug. When I clambered out, the air around me seemed almost warm.

To some, it might sound like the morning schedule of a deranged masochist. But this sequence of cold, hot and then cold again made me feel strangely alive. A sauna followed by an ice bath is known as contrast therapy in sports medicine – and it feels surprisingly good. When I do it, I experience a rush of energy, which gives way to a mood of happiness.

It's something I have long enjoyed, like millions of other Finns who regularly take part in the traditional sauna bathing rituals. It is such an important part of life in Scandinavia that Sweden's entry to this year's Eurovision Song Contest – performed by Kaj, whose members hail from Finland – is an ode to the sweaty heat of saunas. The song "Bara Bada Bastu", which translates roughly as "just take a sauna", had been among the favourites in the competition and had been topping the charts ahead of the competition. In the end the performance came in fourth place.

To those outside of Finland the lurch from cold, to hot and to cold again can seem extreme, but it is a health craze that is now spreading beyond Scandinavian borders to other parts of the world.

So I wanted to delve into the science behind the Finnish sauna and find out whether it really is good for me.

"Finns go to the sauna at least once a week and spend between five to 30 minutes in about 80-100C (176-212F) heat," says Heikki Junkkari, a doctor who sits on the board of the Finnish Sauna Society, an organisation that has promoted Finnish sauna culture since 1937.

Saunas have existed for thousands of years and going to them is a deeply-rooted part of Finnish life. It unites Finns across generations and regions. Saunas have even made it onto Unesco's Intangible Heritage List.

There are an estimated 3.3 million saunas in this country of 5.5 million people, meaning they are accessible to almost everyone. Sauna users often cool off in an icy lake or river or take a cold shower. People typically alternate between hot and cold several times, says Junkkari.

But going from heat to ice is an intense experience that affects blood circulation in the human body, says Setor Kunutsor, a cardiovascular epidemiologist at the University of Manitoba.

"When you go to the sauna, the temperature inside the body goes up slowly from 37C (99F) to as high as 39C (102F). Blood vessels dilate, you start sweating and that reduces blood pressure," he says. "Being immersed in cold water does the opposite: blood vessels contract and blood pressure goes up."

Many medical papers have discussed the potential health benefits and risks of saunas and cold baths, but Junkkari warns that many of these studies lack scientific quality.

"Very few studies are good quality randomised controlled trials," he says, referring to a scientific methodology in which participants in a study are randomly sorted into groups in order to test the effectiveness of a drug, activity or some other........

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