Sperm Swim Faster in the Summer

Summer may be the best season for sperm.

A team from the University of Manchester, Queen’s University (Ontario, Canada), and Cryos International (Aarhus, Denmark), analyzed more than 15,000 men in Denmark and Florida and found that sperm motility peaked in summer and declined in winter. The pattern appeared regardless of temperature.

Why study seasonal variation in sperm quality?

Human biology changes with the seasons. Hormone levels shift, sleep patterns change, and mood and metabolism follow annual cycles. Reproduction is no exception, yet scientists still do not fully understand how male fertility varies across the year.

One of the most important features of sperm quality is motility—the ability of sperm to swim forward. This movement is needed for sperm to reach and fertilize an egg; higher motility is linked to higher chances of pregnancy.

Studies examining seasonal patterns in sperm quality have reached conflicting conclusions. Some have reported higher motility in winter, while others have found the opposite, with peaks in warmer months. Many of these studies were small or limited to one country, and laboratory methods also differed, making comparisons difficult.

Temperature has often been proposed as the main driver. Sperm production takes ~74 days and is known to be sensitive to heat. Large studies have linked higher temperatures to poorer sperm quality; however, these findings have not been consistent across locations or populations.

These inconsistencies have left scientists to wonder: are seasonal changes in sperm driven by temperature itself, or by other seasonal factors such as daylight, lifestyle, or behavior?

The new study analyzed sperm samples from men living in two very different climates to find out if sperm quality changes through the year—and what might be behind it.

Seasonal changes in sperm quality

The team used samples from 15,581 men aged 18–45 years who applied to become sperm donors between 2018 and 2024. Of these, 10,670 lived in Denmark, and 4,911 lived in Florida. The two locations differ greatly in climate, with Denmark having cold winters and mild summers, while Florida stays warm all year.

Each sample was analyzed within one hour, with the team measuring sperm concentration, how well the sperm moved, the total number of swimming sperm, and semen volume. They also examined temperature data from the month of collection and from two months earlier, when sperm development begins. Advanced statistical models separated seasonal effects from age, location, and long-term trends.

The researchers found a clear seasonal pattern in sperm motility. Fast-moving sperm were most abundant in June and July, and lowest in December and January. This pattern appeared in both Denmark and Florida.

“We were struck by how similar the seasonal pattern was in two completely different climates,” said co-author Dr. Allan Pacey, a professor of andrology at the University of Manchester. “Even in Florida, sperm motility still peaked in summer and dipped in winter, which tells us that ambient temperature alone is unlikely to explain these changes.”

Temperature during sperm development showed only weak or inconsistent effects.

Other sperm measures did not follow this pattern. Total sperm concentration and semen volume remained stable throughout the year, suggesting sperm production itself did not change seasonally, but sperm function did.

Age also impacted the results, with sperm motility being highest between the ages of 25–35 years old.

The team also found interesting long-term changes; motility declined in Denmark between 2019 and 2022, while it increased in Florida over the same period—which they theorize could be linked to COVID lifestyle changes.

What seasonal sperm changes mean for fertility testing

The findings could influence how fertility tests are interpreted.

“Our study highlights the importance of considering seasonality when evaluating semen quality. It also shows that seasonal variation in sperm motility occurs even in warm climates. These findings deepen our understanding of male reproductive health and may help improve fertility outcomes,” said Pacey.

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Clinics may need to take the season into account when assessing sperm. A sample collected in winter could look different from one collected in summer.

“These data suggest that the month of the year when a man first attends a clinic to be evaluated as a sperm donor will impact the quality of the sample he produces and therefore may influence the chances of him being accepted as a donor,” said co-author Dr. Anne‑Bine Skytte, chief medical officer at Cryos International. “Having an ejaculate that contains a high number of swimming sperm is one of the main characteristics we look for when deciding whether he is suitable or not.”

The study also raises deeper biological questions. Since temperature alone does not explain the seasonal pattern, other factors have to play a role, such as daylight exposure, physical activity, diet, or environmental exposures.

However, the study has its limits. The participants were sperm donor applicants, who may not represent the general population. Information on lifestyle, health, or behavior was also not included, which can strongly affect sperm quality.

The researchers also noted very large differences between individuals, showing that seasonal effects are small compared with natural variation between men.

Future studies will need to examine more diverse populations and include lifestyle data. More studies are needed to understand the biological mechanisms at play, such as hormonal cycles and the effects of daylight.

Reference: Lassen E, Pacey AA, Skytte AB, Montgomerie R. Seasonal trends in sperm quality in Denmark and Florida. Reprod Biol Endocrinol. 2026. doi: 10.1186/s12958-026-01537-w

This article is a rework of a press release issued by the University of Manchester. Material has been edited for length and content.