There is no question that middle-aged men in Eastern Finland that use the sauna more also live longer. There is, however, a lot of debate re: whether the sauna use causes it or something else. The "something else" might be cardiovascular fitness & conditioning. Is it that having CV fitness increases your sauna use by putting you in the gym more, and making it possible for you to remain comfortable in 175 degrees F heat?

So is it the heat shock proteins and FOXO3, as stated by Rhonda Patrick? or the association with fitness as stated by multiple letters to the editor of JAMA, who published a key article on this in 2015? Here's the 2015 article:

https://jamanetwork.com/journals/jamainternalmedicine/fullarticle/2130724

My hunch is that it's both. The same articles published a follow-up analysis article in 2018. Unfortunately, the follow-up analysis wasn't published in JAMA Internal Medicine. I'm thinking that's because this analysis (multivariate analysis) should have been done on the 2015 study, but wasn't. Whatever happened re: the submissions and editorial judging, the follow-up indicates my hunch is right:

https://pubmed.ncbi.nlm.nih.gov/29551418/