Sperm quality is at its peak in the summer, study finds

A new study suggests that human sperm quality follows a seasonal pattern, peaking in the summer and dipping in the winter.

The research, published Feb. 21 in the journal Reproductive Biology and Endocrinology, analyzed semen samples from more than 15,000 sperm donors in Denmark and the U.S., specifically the state of Florida. Across both populations, the scientists found a consistent pattern: the highest levels of progressively motile sperm — sperm that can swim efficiently in a straight line — appeared in June and July, while the lowest levels occurred in December and January.

Seeing this pattern, "we wondered if men might have more chance of being accepted as a [sperm] donor if they apply in the summer," study co-author Allan Pacey, a professor of andrology at the University of Manchester, told Live Science in an email. "By extension, it may also mean that couples in Denmark and Florida who want to try for a baby might do better in the summer. But that is just a theory."

​​Dr. Sherman Silber, a urologist and director of The Silber Infertility Center of St. Louis, who was not involved in the study, doesn't think the seasonal variation in sperm motility is likely to affect real-world fertility. The differences reported in this study are "very, very tiny" and "make no difference whatsoever biologically," he told Live Science in an email.

Lifestyle influence or evolutionary relic?

To explore sperm quality over time, the researchers analyzed semen samples from 15,581 men ages 18 to 45 who applied to be sperm donors between 2018 and 2024. Samples were collected in four Danish cities — Aarhus, Aalborg, Odense and Copenhagen — as well as in Orlando, Florida.

The team used computer-assisted sperm analysis to measure semen volume, sperm concentration and the number of progressively motile sperm in each sample. (Progressively motile sperm swim in straight lines or large circles, while non-progressively motile sperm move in tight circles but do not travel forward.)

Because sperm take about 74 days to develop in the body, the researchers also examined whether temperatures in the weeks leading up to ejaculation might influence sperm quality. But they found little evidence that either temperatures at the time of ejaculation or those from two months prior had any measurable link to sperm quality.

That said, temperature may indirectly influence sperm quality by influencing lifestyle factors known to affect sperm quality, the researchers hypothesized.

"The fact that the seasonality still existed when we accounted for ambient temperature made us think that other lifestyle changes might be important," Pacey said. "This could include diet, exercise, exposure to sunlight. But we did not measure these things so we can only speculate."

Silber thinks the seasonal pattern may be an evolutionary vestige. In many animals living in temperate climates, reproduction is timed so that offspring are born in spring, when conditions are more favorable and resources more abundant. If sperm quality peaks in summer, that timing could increase the likelihood of spring births. In humans, however, this seasonal effect is likely muted since humans have adapted to survive well in the winter, Silber proposed.

Different studies, different trends

Previous studies have also reported seasonal changes in sperm quality, but the documented patterns were inconsistent. Some research, including a study from Italy, also found peak sperm motility during the summer, in line with the new findings.

But other studies have reported the opposite trend. For instance, a large analysis of more than 21,000 semen samples from southern China found that sperm motility peaked in late winter and declined through the summer months. The differences between these studies hint that regional lifestyle and environmental factors, such as differences in temperature, humidity, or social behaviors, may shape how sperm quality fluctuates over time, the study authors wrote in the new paper.

In addition to seasonal variation, the new research found strong links between sperm quality and age. Sperm motility was highest among men in their 30s, and lower in men younger than 25 and older than 40.

The researchers also observed a significant dip in sperm quality in Denmark between 2019 and 2022, followed by a rebound in 2023, possibly reflecting lifestyle changes during the COVID-19 pandemic. For instance, lockdowns altered the general public's working patterns, diet, and physical activity levels, and previous studies suggest that each of these factors can influence sperm motility.

In contrast, sperm quality in Orlando increased gradually from 2018 to 2024, a trend that remains unexplained and warrants further study, the study authors noted.