In Finland, there are more saunas than cars.

3.3 million saunas for a population of 5.5 million. They've been using heat therapy for over a thousand years. And now modern science is catching up.

Heat saves lives.

The most cited sauna study followed over 2,300 Finnish men for 20 years. Men who used the sauna 4 to 7 times per week had a 40 percent lower risk of all-cause mortality compared to once a week. They also had 63 percent lower risk of sudden cardiac death and 50 percent lower risk of cardiovascular disease. From my research into longevity interventions, these are reductions that would make front-page news if they came from a drug.

So what's happening in there?

When your core temperature rises by one to two degrees Celsius -- about 15 minutes at 80 to 100 degrees -- your body launches a cascade of protective responses.

Heat shock proteins. Molecular chaperones that repair damaged proteins and protect cells from future stress. Your body only produces them in significant quantities under heat stress.

Cardiovascular conditioning. Heart rate rises to 100 to 150 bpm -- comparable to moderate exercise. Blood vessels dilate, blood flow increases. Regular use reduces hypertension risk by nearly 50 percent.

Immune activation. A single session increases white blood cell counts. Regular use means fewer colds and respiratory infections. Your body treats heat the same way it treats a fever -- by ramping up immune surveillance.

Brain protection. The same Finnish study showed 65 percent reduced risk of Alzheimer's in frequent sauna users. Heat stress increases BDNF, which supports neuroplasticity and new neural connections.

The protocol.

15 to 20 minutes per session at 80 to 100 degrees Celsius. Traditional and infrared saunas both work -- infrared operates at lower temperatures but penetrates deeper into tissue.

Frequency matters more than duration. Four sessions per week is where the mortality data gets dramatic. Two or three still delivers significant benefits, but four to seven is the target.

Hydrate before and after. You'll lose 300 to 500 millilitres of sweat per session. Electrolytes matter -- not just water.

The contrast between sauna and cold exposure -- heat followed by cold -- amplifies the benefits of both. The Scandinavian tradition of alternating between sauna and cold plunge isn't just tradition. It's hormesis at its finest.

Sauna pairs naturally with the cold exposure protocol and the immune-supporting supplements in the Immunity journey. Heat and cold together build a body that adapts to anything.