How Heat Impacts Sperm: New Science Unlocks Clues to Male Fertility

Written by: Mate Health Clinical Education Team
Updated: July 2026

Why Testicles Are Outside the Body

The testicles sit in the scrotum, where they are normally maintained a few degrees cooler than core body temperature. This temperature difference supports normal sperm production.

Researchers have known for decades that frequent or prolonged testicular heat exposure may affect sperm production and certain semen parameters. Human studies involving repeated sauna or hot-water exposure have reported temporary changes in sperm count, motility, DNA packaging, and other measurements. [1,2]

A 2025 laboratory study added a new piece to that picture by identifying a temperature-sensitive mechanism involving CatSper, a calcium channel essential to sperm function. [3]

The finding is scientifically important, but it does not establish an exact temperature at which everyday heat exposure begins to harm human fertility.

What Researchers Discovered in 2025

Researchers led by Dr. Polina Lishko published a study in Nature Communications examining how temperature affects CatSper activity in mouse sperm. [3]

CatSper is an ion channel found in sperm cells. It allows calcium to enter the sperm tail, helping control the change in movement that sperm need as they approach an egg.

The researchers recorded electrical activity directly from individual mouse sperm cells while changing the surrounding temperature. They found that CatSper activity increased sharply around 33.5°C.

The study also found that spermine, a compound naturally present in semen, could inhibit this temperature-sensitive activity. The researchers proposed that this may help protect sperm from activating too early.

The study was conducted in mouse sperm under laboratory conditions. It did not measure pregnancy outcomes, human semen quality, hot-tub use, laptop exposure, varicoceles, or mailed semen specimens.

What CatSper Does

Sperm do not swim in exactly the same way throughout their journey.

Earlier in the reproductive tract, sperm generally move with a more regular swimming pattern. Closer to the egg, they develop a stronger, whip-like movement known as hyperactivated motility. This movement helps sperm navigate the female reproductive tract and interact with the layers surrounding the egg.

Calcium entering through CatSper helps control that transition.

Because hyperactivated movement is normally required later in the sperm's journey, researchers are interested in how CatSper activation is timed. The 2025 study suggests that temperature may be one part of that regulation in mice. [3]

It does not prove that common heat exposure causes human sperm to become prematurely activated or exhausted.

What the 33.5°C Threshold Means

The researchers observed a thermal threshold of approximately 33.5°C in freshly isolated mouse sperm under controlled laboratory conditions. [3]

That number should not be treated as:

• A precise danger threshold for human testicles
• A maximum safe temperature for a hot tub or sauna
• A cutoff for laptop or workplace heat exposure
• A transport standard for human semen specimens
• A temperature that independently predicts infertility

Human and mouse sperm share important biological features, including CatSper, but findings in an animal model do not always translate directly to clinical outcomes in men.

The study provides a possible mechanism for further research. It does not replace human studies evaluating heat exposure and fertility.

What Human Research Says About Heat and Sperm

Human research suggests that repeated, substantial testicular heat exposure may temporarily affect sperm production and certain semen parameters.

In one small study, healthy men completed repeated Finnish sauna sessions over three months. Researchers observed temporary changes in sperm count, motility, mitochondrial function, sperm DNA packaging, and genes involved in heat stress. Measurements moved back toward baseline after the sauna exposure ended. [1]

Another study involving infertile men with repeated hot-tub or hot-bath exposure found that some participants experienced improved semen quality after stopping the exposure. Not every participant responded, and the study did not prove that wet heat caused their infertility. [2]

A larger prospective study followed 3,041 couples and examined male heat exposures in relation to fecundability, the per-cycle probability of conception. Frequent hot-bath or hot-tub use and recent fever showed weak inverse associations with fecundability. [4]

The clearest signal appeared when multiple heat exposures accumulated. Men reporting four or more heat exposures had lower fecundability than men reporting none, with a stronger association among men age 30 or older. The estimates were imprecise and do not prove that the exposures caused the reduction, but the findings suggest that cumulative exposure may matter more than any one modest source of heat. [4]

The overall evidence supports reasonable caution around frequent or combined heat exposures, particularly for men who already have abnormal semen parameters. It does not show that occasional warmth causes infertility in otherwise healthy men.

Common Sources of Heat Exposure

The strength of evidence varies by exposure.

Exposure	What the evidence suggests
Frequent hot tubs or saunas	Human studies suggest repeated high-heat exposure may temporarily affect certain semen measurements
Fever	A substantial fever may temporarily affect sperm production and may be associated with a lower probability of conception
Multiple combined heat exposures	Cumulative exposure was associated with lower fecundability in a large prospective cohort, particularly among men age 30 or older
Occupational heat	Prolonged workplace heat has been associated with poorer semen quality in some studies
Laptop directly on the lap	Can increase local temperature, but direct evidence of reduced fertility is limited
Prolonged sitting or heated seats	Evidence for either exposure alone is limited; repeated combined exposures may be more relevant
Tight underwear	May modestly affect scrotal temperature, but evidence for pregnancy outcomes is inconsistent
Varicocele	A medical condition associated with infertility in some men; increased temperature is one of several proposed mechanisms

These exposures should not be assigned universal high, moderate, or low risk ratings. Frequency, duration, baseline semen quality, age, underlying health, and the number of exposures all matter.

Heat, Varicoceles, and Male Fertility

A varicocele is an enlargement of veins within the scrotum. Varicoceles are common and do not cause infertility in every man.

When a varicocele affects fertility, possible mechanisms include increased testicular temperature, oxidative stress, altered blood flow, hormonal effects, and impaired testicular function. Heat is one possible part of the explanation, but CatSper temperature gating has not been established as the reason varicocele treatment may improve semen parameters.

AUA/ASRM guidance states that varicocele repair may be considered in men who are trying to conceive and have a palpable varicocele, infertility, and abnormal semen parameters. [6,7]

Men who notice scrotal heaviness, a dull ache, enlarged veins, testicular asymmetry, or fertility concerns should discuss an examination with a physician or reproductive urologist.

What the Discovery Could Mean for Future Research

CatSper is highly specific to sperm, making it an interesting potential target for future fertility treatments and non-hormonal male contraception.

Researchers may eventually study whether CatSper activity can be increased, reduced, or timed differently to influence sperm function. However, the 2025 study did not test a medication, fertility treatment, or contraceptive in humans.

Any clinical application remains experimental.

How Heat Connects to Semen Testing

The CatSper study should not be used as evidence that a particular shipping system prevents heat-triggered activation in human sperm.

The stronger connection is more straightforward: semen specimens are sensitive to pre-analytical conditions, including time, temperature, handling, and specimen stability before laboratory analysis.

Mate Health's Active Cooling transport method was evaluated across 222 live specimens. The study compared baseline measurements with results after 24 to 34 hours of controlled cooled transport. Strong agreement was demonstrated across ten evaluated semen parameters, with R² values ranging from 0.90 to 0.98. [8]

That validation supports the Mate Health transport method under its defined collection and shipping conditions. It does not establish that every mail-in specimen system preserves samples equally.

A recent fever or repeated heat exposure may also affect a specimen before it is collected. Mention significant recent illness or unusual heat exposure when reviewing results with a physician.

What You Can Do

The human research suggests focusing on repeated and cumulative exposure rather than treating any single everyday source of warmth as decisive. [4]

Reasonable steps include:

• Limit frequent or prolonged hot-tub and sauna use when actively trying to conceive, particularly if semen parameters are already borderline.
• Reduce avoidable combinations of heat exposure, such as frequent hot tubs, prolonged heated-seat use, and extended laptop use directly on the lap.
• Use a desk or lap desk during long laptop sessions. Laptops can raise local temperature, although direct fertility evidence is limited.
• Choose comfortable, breathable clothing if excessive warmth is a concern. Underwear type alone is unlikely to determine fertility.
• Break up prolonged sitting for general health, while recognizing that evidence of a direct fertility benefit is limited.
• Seek evaluation for possible varicocele symptoms, including scrotal pain, heaviness, enlarged veins, or testicular asymmetry.
• Tell your physician about a significant recent fever when reviewing semen-analysis results.

The complete process of making new sperm, called spermatogenesis, requires about 90 days to complete. In practical terms, a significant illness or exposure may be reflected in semen measurements over about three months. [5]

The appropriate timing for repeat testing depends on the original results, the exposure, and a physician's recommendation.

FAQs

Is There a Specific Temperature That Harms Human Sperm?

No single clinical cutoff has been established for everyday human exposure. The 2025 study identified a CatSper threshold of approximately 33.5°C in mouse sperm under laboratory conditions. That figure is not a human safety threshold. [3]

Do Hot Tubs and Saunas Lower Sperm Count?

Repeated hot-tub or sauna exposure has been associated with temporary changes in sperm count, motility, and other semen measurements in small human studies. Frequent hot-bath or hot-tub use was also weakly associated with lower fecundability in a large prospective study. Occasional exposure is less clearly associated with fertility problems. [1,2,4]

Do Several Heat Exposures Matter More Than One?

Possibly. In a prospective study of 3,041 couples, the clearest reduction in fecundability occurred among men reporting several heat-related exposures, particularly those age 30 or older. The findings were observational and imprecise, so they do not establish a precise safe or harmful number of exposures. [4]

Does Keeping a Laptop on Your Lap Affect Fertility?

A laptop directly on the lap can raise scrotal temperature, particularly during prolonged use. Evidence that laptop use by itself causes reduced fertility remains limited. Using a desk or lap desk is a simple way to reduce direct heat.

Does Tight Underwear Affect Sperm?

Some research has associated tighter underwear with differences in scrotal temperature or semen measurements, but evidence for pregnancy outcomes is inconsistent. Underwear type is unlikely to be a major cause of infertility by itself.

Should I Delay Semen Testing After a Fever?

A significant fever may temporarily affect sperm production. Because the complete process of making new sperm requires about 90 days, a physician may recommend waiting before repeat testing. The appropriate timing depends on the illness and reason for testing.

Is the 2025 Research Relevant to At-Home Semen Testing?

Only indirectly. The study shows that sperm biology can be temperature-sensitive, but it did not evaluate human specimen transport. At-home transport methods should be supported by direct analytical validation of the collection and shipping system.

Could CatSper Lead to a Male Contraceptive?

CatSper is being studied as a potential target because it is highly specific to sperm. However, this remains early-stage research, and the 2025 study did not test a contraceptive in humans.

Medical Disclaimer

This article summarizes published research for educational purposes and does not constitute medical advice. The CatSper study discussed was conducted in mouse sperm under laboratory conditions, and animal findings do not always translate directly to human fertility outcomes.

Consult a physician or reproductive urologist if you have fertility concerns, possible varicocele symptoms, persistently abnormal semen parameters, or questions about recent fever or heat exposure.

Curious what your results show? Explore Mate Health's at-home semen analysis options, shipped with Active Cooling transport packaging and processed in a CLIA-certified, CAP-accredited high-complexity andrology laboratory using WHO 6th Edition (2021) methodology.

References

1. Garolla A, Torino M, Sartini B, et al. Seminal and molecular evidence that sauna exposure affects human spermatogenesis. Hum Reprod. 2013;28(4):877-885. doi:10.1093/humrep/det020. PMID: 23411620.
2. Shefi S, Tarapore PE, Walsh TJ, Croughan M, Turek PJ. Wet heat exposure: a potentially reversible cause of low semen quality in infertile men. Int Braz J Urol. 2007;33(1):50-57. PMID: 17335598.
3. Swain DK, Vergara C, Castro-Arnau J, Lishko PV. The essential calcium channel of sperm CatSper is temperature-gated. Nat Commun. 2025;16:3657. doi:10.1038/s41467-025-58824-0. PMID: 40246904.
4. McKinnon CJ, Joglekar DJ, Hatch EE, et al. Male personal heat exposures and fecundability: a preconception cohort study. Andrology. 2022;10(8):1511-1521. doi:10.1111/andr.13242. PMID: 35924639.
5. Amann RP. The cycle of the seminiferous epithelium in humans: a need to revisit? J Androl. 2008;29(5):469-487. doi:10.2164/jandrol.107.004655. PMID: 18497337. (Reconciles the duration of spermatogenesis with epididymal transit time.)
6. American Urological Association; American Society for Reproductive Medicine. Diagnosis and Treatment of Infertility in Men: AUA/ASRM Guideline, Part II. 2020; amended 2024.
7. Brannigan RE, Hermanson L, Kaczmarek J, Kim SK, Kirkby E, Tanrikut C. Updates to Male Infertility: AUA/ASRM Guideline (2024). J Urol. 2024;212(6):789-799. doi:10.1097/JU.0000000000004180. PMID: 39145501.
8. Mate Health. Analytical Validation of the Mate Health At-Home Semen Collection and Transport System. Analytical validation report. 2026.