Heat and sperm: what the studies actually say

The testes sit in the scrotum for a reason. Sperm production works best a few degrees below core body temperature, and the scrotum has several ways of keeping it there: thin skin that sweats easily, muscles that raise or lower the testes with heat and cold, and blood vessels arranged so that incoming blood is cooled by outgoing blood.

None of this is controversial. It is textbook physiology and appears in patient information from health services such as Sweden's 1177. What matters for regular sauna users is what happens when that regulation meets 80–90 degrees, regularly, over a long period.

One key detail is the timescale. A sperm cell takes roughly two and a half months to develop. So what you do today shows up in a sample weeks later, and any change also takes that long to reverse.

We are not doctors and we do not run our own research. We read published studies and reviews and ask the same simple questions every time.

• How many participants, and who were they?
• Was there a control group, and was measurement blinded?
• What was measured: lab values in a sample, or real outcomes like pregnancy?
• Did the effect reverse, and how long did it take?
• Has anyone else replicated the result?

1. The most cited sauna study (Garolla et al., 2013) followed ten healthy men in their twenties and thirties. They used a Finnish sauna at 80–90 °C for 15 minutes per session, twice a week, for three months.
2. During that period sperm count and motility fell measurably from baseline. The researchers also saw cell-level changes suggesting the heat affects production itself.
3. The effect was temporary. After the men stopped, values recovered, but only after about six months. That fits the length of the production cycle.
4. A larger Harvard observational study (Mínguez-Alarcón et al., 2018) of more than 650 men found that those who mostly wore loose underwear had higher sperm concentration than those who wore tight styles. It is about everyday clothing rather than the sauna, but it points the same way: local heat matters.
5. A 2026 narrative review in Translational Andrology and Urology sums up the field: garment fit and local temperature are reasonable factors to consider, but intervention studies showing effects on real outcomes are largely missing.

The principle that heat affects sperm production is strongly supported. That the sauna specifically causes a measurable but temporary effect is supported by one small, well-run study. How large the effect is for any individual, at other temperatures and session lengths than in the study, we do not know.

Ten participants is few. The study was not blinded and had no separate control group; the men served as their own controls. It measured lab values, not whether anyone had more difficulty conceiving. It shows a direction, not an effect size you can plan around.

On our Science page we grade every claim from A (strong evidence) to D (anecdote). This question lands at B for the principle and C for the sauna specifically.

For most sauna users this is about awareness, not worry. If you are not planning children in the near future, it is reasonable to keep using the sauna as usual.

If you and a partner are trying to conceive, or you are going through a fertility assessment, it makes sense to mention your sauna habits to your doctor. Swedish medical literature has noted that the male side of assessments is often incomplete, and lifestyle factors like heat belong in that conversation.

Other factors probably matter more: smoking, alcohol, excess weight, sleep and some medications. Mayo Clinic and national health services list them. Heat is one factor among several.

The sauna is not the only source of local heat. Hot tubs and hot baths have been studied in small samples with similar results: regular exposure to hot water coincided with lower values, which often improved once the habit stopped. The evidence is about as thin as for the sauna.

Everyday factors appear in the literature too: prolonged sitting, a laptop on the lap, tight underwear and work in hot environments. Each is weakly studied. Together they point to the same principle as the sauna study: scrotal temperature matters over time.

Fever is a clear example. A spell of high fever can show up in a semen sample several weeks later, and values then recover. Same mechanism, same timescale.

Sperm production is a multi-stage process that takes around 70 days, followed by further days of maturation. A sample taken today therefore reflects conditions two to three months ago.

That explains why the sauna study needed three months of exposure to see a clear effect, and six months to see recovery. It also explains why single sauna sessions are unlikely to decide anything: the pattern over weeks and months is what counts.

If you want to change a habit, the lesson is simple. Do not expect a difference after a week, and do not draw conclusions from a single sample.

• "The sauna makes you sterile." No. The studies show a temporary drop in lab values in healthy men, not sterility.
• "One session is enough to cause harm." There is no support for single sessions having a lasting effect.
• "A cold plunge afterwards cancels it out." This has not been studied. A plunge after the round does not change the fact that the heat already acted during it.
• "This only applies to the sauna." The principle applies to local heat in general. The sauna is just the most studied situation.
• "If my values are normal it does not matter." For most people that is true. The question mainly matters for those trying to conceive.

If you are going through an assessment, it helps to have answers ready to a few questions. It makes the conversation shorter and more useful.

• How often do you use the sauna, for how long and at what temperature?
• Do you use hot tubs, hot baths, or sit for long periods with heat against your body at work?
• Have you had a fever in the past three months?
• Which medications and supplements do you take?
• What do sleep, alcohol and exercise look like in a normal week?

• Whether shorter rounds, lower temperatures or fewer sessions per week have a smaller effect. This has not been tested systematically.
• Whether the effect differs between dry sauna, steam room and infrared sauna.
• Whether the results apply to older men or men with already low values.
• Whether local cooling during or after a round changes anything at all. No controlled studies exist.
• How much the sauna matters compared with other lifestyle factors in a real population.

We think the question deserves to be taken seriously without being dramatised. We use the sauna most days ourselves and plan to keep doing so.

We make no claim that anything we sell affects fertility. We summarise the research because it is interesting and because frequent sauna users deserve an honest overview.