Yes, and it is one of the more reversible factors. Sperm production works best a couple of degrees below core body temperature, which is why the testes hang outside the body. In a 2013 study, two 15-minute sauna sessions a week for three months cut sperm count and motility markedly in ten healthy men, and every measure was back to baseline six months later. A 2005 study found a laptop on the lap raised scrotal temperature by 2.6 to 2.8°C, and a 2018 study of 656 men found boxer wearers had 25% higher sperm concentration than men in tighter underwear. Because one cycle of sperm production takes about 74 days, recovery after removing the heat takes roughly three to six months.
Saunas
The clearest experiment is Garolla and colleagues, 2013. Ten healthy men with normal semen used a Finnish sauna (80 to 90°C) for 15 minutes twice a week for three months. Sperm count and progressive motility fell markedly during the exposure, with changes in sperm DNA packaging and mitochondrial function alongside, and all of it returned to baseline by the six-month follow-up. Ten men is a small study, but it is a controlled one with a recovery arm, which is rare in this field, and it is the reason saunas sit near the top of most clinics' lists of things to pause while trying to conceive.
Hot tubs and hot baths
A 2007 pilot study at UCSF followed 11 infertile men who had been immersing themselves in a hot tub, jacuzzi or hot bath for 30 minutes or more a week for at least three months. After stopping for three to six months, five of the 11 improved, with average motility rising from 12% to 34% and total motile sperm counts up several-fold. The six who didn't improve were mostly chronic smokers, which the authors flagged as a possible reason. Eleven men and no control group make this a signal rather than a proof, but the direction matches the sauna study and the physiology.
Laptops
Sheynkin and colleagues (2005) measured scrotal temperature in 29 healthy men sitting with a working laptop on their laps. Temperature rose by 2.8°C on the right and 2.6°C on the left, against a 2.1°C rise from sitting in the same closed-leg position without a laptop. Two things follow: the posture alone warms the testes, and the laptop adds to it. The study measured temperature, not sperm, so the link to semen quality is by way of the heat studies above rather than direct. A desk, a lap tray, or simply sitting with the knees apart removes most of it.
Underwear
Mínguez-Alarcón and colleagues (2018) looked at 656 men attending a fertility center and compared the ones who mostly wore boxers with the ones who mostly wore briefs, bikinis, jockeys or other tighter styles. Boxer wearers had 25% higher sperm concentration and 17% higher total sperm count, along with 14% lower FSH, a pituitary hormone that rises when the testes are working harder. Motility and morphology didn't differ. It's a cross-sectional study of men at a fertility clinic, so it can't prove that switching underwear would change your count, but the hormone finding makes a simple heat explanation more plausible. It is also the cheapest change on this page.
Cycling
Cycling combines heat, pressure and tight clothing, and the one large study is Wise and colleagues (2011): 2,261 men attending a fertility clinic. Men who bicycled five or more hours a week had roughly twice the odds of a low sperm concentration (odds ratio 1.92) and of a low total motile count (odds ratio 2.05) compared with men who didn't cycle. Regular exercise in general, including other kinds of vigorous activity, wasn't associated with any semen measure. The finding is specific to a lot of cycling, not to cycling: a commute or a weekend ride doesn't reach the five-hour band. If you ride more than that, fewer hours or a different mix of training is the lever, not stopping exercise. The exercise section of the main guide covers the rest.
Sitting
The laptop study's control condition, sitting with the legs together and no laptop, raised scrotal temperature by about 2°C on its own. That is a temperature measurement over a short session, not a semen study, so the honest summary is that long stretches of sitting warm the testes and nobody has measured how much that matters for sperm. The practical response is breaking up long stretches of sitting rather than anything drastic, which is also what the app's onboarding asks about.
How long recovery takes
Six months in the sauna study; three to six months in the hot tub study. Both fit the biology: one cycle of sperm production is about 74 days, plus one to two weeks in the epididymis, and the cells that were damaged by heat have to be replaced by cells that developed after the exposure stopped. The how-long guide goes through this, and the timeline tool gives you the dates.
What to do
- Pause saunas, steam rooms and hot tubs while you're trying to conceive. Warm showers and baths at body temperature are fine.
- Put the laptop on a desk or a lap tray. If it has to be on your lap, sit with your knees apart.
- Switch to looser, breathable underwear. It is the cheapest change with a study behind it.
- If you cycle five or more hours a week, trade some of those hours for other training.
- Break up long stretches of sitting: stand up every hour or so.
- Give it three to six months, then retest. The sauna study's recovery took that long.
Related guides
Sources
- Garolla A, et al. Seminal and molecular evidence that sauna exposure affects human spermatogenesis. Human Reproduction, 2013.
- Shefi S, et al. Wet heat exposure: a potentially reversible cause of low semen quality in infertile men. International Braz J Urol, 2007.
- Sheynkin Y, et al. Increase in scrotal temperature in laptop computer users. Human Reproduction, 2005.
- Mínguez-Alarcón L, et al. Type of underwear worn and markers of testicular function among men attending a fertility center. Human Reproduction, 2018.
- Wise LA, et al. Physical activity and semen quality among men attending an infertility clinic. Fertility and Sterility, 2011.
- Heller CG, Clermont Y. Spermatogenesis in man: an estimate of its duration. Science, 1963.