Thanks for admitting me into this group and conversation. Having had a sauna in my home for 20+ years I am well-versed in the benefits. I don't refute that sitting in the sauna is deeply quieting. However, when I put on my scientist-hat, I question whether sauna itself directly initiates the parasympathetic (PSNS) response. I think it's a downstream effect, though I think it's mediated by a sympathetic (SNS) response that's expressed as the freeze part of fight/flight/freeze. In a freeze response, there can be stillness, slower breathing and a sense of calm. The slight uptick in SNS activation that is associated with heat exposure (at essence, a stressor) likely catalyzes the PSNS activation through hormesis, which is one of the main reasons I sauna 5+ days per week. I'm not debating the result, though I'm unsure of the causal pathway. I'm curious to hear others' thoughts! The role of intention, ritual, set & setting add additional variables add even more to the discussion.

@John Pederson - yes, let's discuss HRV - lots there, particularly when combined with subjective measures and/or other stress biomarkers. My background; I'm adjunct faculty at the University of Wisconsin - Madison and a collaborator at the Center for Healthy Minds at the UW - Madison.