The majority of sauna research focuses on its effects in people with cardiovascular disease. In the short term, sauna can lower blood pressure and improve measures of endothelial function and arterial stiffness (e.g., flow-mediated dilation and pulse-wave velocity). Over long durations, higher frequencies of sauna use have also been associated with 40%–70% reductions in the risk of all-cause mortality, cardiovascular mortality, hypertension and stroke.  Sauna (particularly Waon therapy) has been frequently studied for treating heart failure and shows promise in terms of reducing its severity (at least in the short term). However, it’s not clear whether these short-term benefits are simply a product of fluid loss (by sweating; fluid overload is a characteristic of certain types of heart failure) or whether they occur through some other, longer-lasting mechanism.
Additionally, sauna may also benefit other conditions, such as dementia/Alzheimer’s disease, respiratory diseases, musculoskeletal disorders (e.g., arthritis), skin diseases, headache, and mental health disorders.
It’s been proposed that sauna may be a strategy for increasing healthspan and lifespan. There is compelling observational research to support this, in addition to mechanistic/non-human/uncontrolled trials on the subject, but direct, high-quality, well-controlled trials in humans are still somewhat lacking.