Few wellness practices have travelled further on less evidence than deliberate temperature exposure. Cold plunge tubs have become fixtures in gyms and in expensive homes, and sauna use is routinely described as a longevity intervention. The two are usually discussed together, which obscures the fact that their evidence bases are quite different in both quantity and quality.

A systematic review published in Frontiers in Physiology in September pooled thirteen studies of cold-water exposure covering about 4,100 participants. Its findings are worth reading carefully, not because they are damning but because they are modest in a way the marketing is not.

From the Lab to the Ledger

The physiological case for both practices is real and well understood. Immersion in cold water triggers an immediate sympathetic nervous system response: blood vessels constrict, heart rate and blood pressure spike, breathing becomes rapid and involuntary. Repeated exposure produces measurable habituation to that shock, and there are plausible mechanisms by which a controlled stressor might produce adaptive benefit, the same general logic that underlies exercise.

Heat works through a different route. Sitting in a sauna raises core temperature and heart rate in a pattern that resembles moderate exercise, dilates blood vessels, and increases plasma volume over repeated sessions. The best-known evidence here comes from large Finnish cohort studies, which found that frequent sauna use was associated with lower cardiovascular mortality.

The word doing the heavy lifting in that last sentence is "associated." Those were observational studies in a country where sauna use is near-universal and woven into ordinary life. People who use a sauna four or five times a week are, on average, people with the time, health and social stability to do so. Controlling for that statistically is not the same as removing it.

The cold-water review is more direct about what the trials show. Pooled estimates for heart rate favoured cold exposure but the confidence intervals crossed zero. Blood pressure showed no consistent benefit, and the systolic estimate shrank towards nothing when the analysis was restricted to controlled studies. Most of the included studies carried high or serious risk of bias, and most outcomes were measured immediately after exposure, so nothing is known about whether any effect persists.

Bio-Pipeline Ledger

Sauna bathing and cardiovascular outcomes: supported by large observational cohorts showing association with lower cardiovascular mortality, with no randomized outcome trial. Widely available and plausibly beneficial rather than demonstrated.

Passive heating and vascular function: small randomized trials show measurable improvements in vascular measures such as flow-mediated dilation. Early evidence on surrogate markers, not on events.

Cold-water immersion for mood and wellbeing: thirteen studies pooled, with high risk of bias, immediate-only measurement and wide confidence intervals. Inconclusive.

Cold-water immersion for cardiovascular measures: no consistent effect on blood pressure, a non-significant signal on heart rate. Not validated.

Cold immersion after resistance exercise: a body of sports science evidence suggests it may blunt some muscle adaptations to training. Worth knowing if the goal is building strength.

The Clinical Reality Check

What is genuinely reasonable to say is that a sauna, for a healthy person who enjoys it, is a pleasant practice with a plausible mechanism and an encouraging but observational evidence base. That is a perfectly good reason to use one. It is not a reason to describe it as a proven intervention, and anyone selling it as such is ahead of the data.

Cold exposure is on weaker ground still, and the review is unusually clear that it cannot establish safety for self-directed use. Acute cold immersion produces a sharp cardiovascular response, which is the point of the practice and also the risk. For someone with coronary disease, an arrhythmia, uncontrolled hypertension, or who is pregnant, that response is not a neutral event. Adverse events were reported inconsistently across the studies, so the absence of recorded harm is not evidence of its absence.

The honest framing for both is that they belong in the category of things that are probably fine and possibly helpful, not in the category of things with demonstrated benefit. That distinction matters mostly because of what gets displaced. Time and money spent on a plunge tub are time and money not spent on the interventions with randomized evidence behind them: regular aerobic exercise, resistance training, blood pressure control, sleep, and not smoking. None of those photograph as well.

If you want to use either, the sensible conditions are unglamorous. Start conservatively, never alone in cold water, and clear it with a clinician first if you have any cardiovascular condition. Treat it as something you do because you like it and it makes you feel better, which is a legitimate reason on its own, rather than as an investment in a longevity outcome that no trial has yet measured.