In 1976, a British Army physician named H. Foster wrote a letter to the British Medical Journal about a pressing issue in European public health: the epidemic of fatal heart attacks among middle-aged Finnish men. In it, he speculated that the reason Finland had one of the highest rates of coronary heart disease in the world was the nation’s sauna.

On the face of it, the hypothesis made sense. A sauna is a stressful environment for the body, with an ambient temperature of 80°C (176°F) and above. The pulses of steam place repeated strain on users’ cardiovascular system. Stay in for too long, and you will surely perish.

But more recent research suggests Foster may have got it entirely wrong, and that frequently partaking in Finnish sauna – as well as other kinds of heated bathing – may significantly reduce the risk of heart attacks, while improving health across the board.

Having grown up mostly in swampy, humid Washington DC, I have never really enjoyed feeling hot and sweaty. But after nearly two decades of writing about health and longevity, watching as data piled up suggesting that limited heat exposure might benefit our bodies and minds, I decided to give sauna a go. I found the research so compelling that I ended up spending three years digging into it and writing a book about the myriad benefits of heat. What’s more, along the way, I discovered why we are so well-primed to reap all these benefits.