Heat bathing exists in many cultures, but the Finnish form has become the global default. Its spread followed migration and social change rather than any health finding.
It began as household infrastructure
In rural Finland the sauna was a small heated building used for washing, laundry, food preparation and childbirth, in a climate where clean hot space was scarce and valuable.
It was ordinary rather than special, present at nearly every dwelling and used routinely, which is a different social position from a facility visited occasionally.
That ubiquity is why the practice survived urbanisation intact. It moved into apartment blocks and workplaces rather than being left behind in the countryside.
Migration carried the form
Finnish emigrants built saunas wherever they settled, and the structures persisted in those communities long after other imported customs had faded.
The design travelled well because it is simple: an insulated room, a stove, stones and benches at different heights. It requires no rare materials and adapts to available space.
Commercial gyms and hotels later adopted it as a low-maintenance amenity, which is how most people outside the Nordic countries first encountered one.
What heat exposure does physically
Core temperature rises, blood vessels near the skin dilate to shed heat, and heart rate increases to maintain circulation through the expanded vessel network.
The cardiovascular pattern resembles moderate exercise in some respects, though without the muscular work, and blood pressure typically falls during and shortly after a session.
Cells also respond to elevated temperature by producing protective proteins that assist other proteins in holding their shape under stress.
The research came from an unusual setting
Long-term studies of sauna use were possible in Finland because the habit is widespread, frequent and stable enough to be recorded across decades.
Those studies report associations between frequent use and cardiovascular outcomes, but they are observational, and frequent users differ from infrequent ones in ways beyond heat exposure.
Associations of this kind establish that the question is worth asking rather than establishing that heat produced the result.
The social function is doing work too
Sauna use in its home context is communal, unhurried and phone-free, occupying a block of time with no other purpose and no screens.
Those features overlap with several things independently associated with wellbeing, which makes them hard to separate from the heat in any observational data.
Anyone with cardiovascular disease, low blood pressure or who is pregnant should check with a doctor before regular heat exposure, since the load is genuine.