Sauna is the recovery modality with the best long-term evidence and it is largely evidence for something other than recovery. Getting that distinction right changes when you would actually use one.
What the evidence actually supports
Heat acclimation. Repeated heat exposure drives plasma volume expansion, improved sweat response and cardiovascular adaptations. For endurance athletes this is a real performance benefit, most obviously when competing in heat but plausibly beyond it.
Cardiovascular association. Large observational work from Finland links frequent sauna use with lower cardiovascular mortality. Worth reading with the usual caution: people who use saunas four times a week differ from people who do not in many ways, and observational data cannot separate those.
It feels good and people keep doing it. Not a trivial point. Adherence beats optimisation, and a recovery habit you actually maintain has a real advantage over a better one you abandon.
What it does not do
Clear soreness reliably. The DOMS evidence for heat is weaker than for cold, and if feeling less sore tomorrow is the goal then cold has the better case.
Detoxify anything. Sweat is overwhelmingly water and electrolytes. Your liver and kidneys handle the job the marketing attributes to a wooden room.
Produce meaningful fat loss. The weight change after a session is water and it returns as soon as you drink.
How to use it
After training, not before. You are extending an already elevated core temperature, which is the stimulus. Before a session it mostly just makes the session worse.
15 to 30 minutes, several times a week. That is the pattern the acclimation research uses. Consistency matters more than any single long session.
Rehydrate properly. Meaningful fluid and electrolyte loss, and turning up dehydrated to tomorrow’s session will cost you more than the sauna gained. It will also show up as a raised overnight heart rate, covered in what is a normal sleeping heart rate.
Infrared is a different stimulus. Lower air temperature, and the heat acclimation evidence largely comes from traditional saunas. Not useless, and not interchangeable.
Choosing between them is cold plunge vs sauna.