Study finds that sperm quality may change depending on the season

New research suggests that human sperm quality follows a seasonal pattern, peaking in the summer and declining in the winter.

The study, published February 21 in the journal Reproductive Biology and Endocrinology, analyzed semen samples taken from more than 15,000 sperm donors in Denmark and the United States, particularly in Florida. Across both populations, scientists found a consistent pattern. This means that levels of progressively motile sperm (sperm that can efficiently swim in a straight line) were highest in June and July, while lowest levels were in December and January.

Seeing this pattern, she said, “I thought maybe men were more likely to be accepted as women. [sperm] “If you apply in the summer, you can give it to donors,” study co-author Alan Pacey, a professor of andrology at the University of Manchester, told Live Science via email. “This, in turn, may mean that Danish-Florida couples looking for a baby may have better luck in the summer.” But that’s just a theory. ”

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Dr. Sherman Silver, a urologist and director of the Silver Fertility Center in St. Louis, who was not involved in the study, believes seasonal fluctuations in sperm motility are unlikely to affect real-world fertility. The differences reported in the study were “very, very small” and “biologically there is no difference,” he told Live Science via email.

Is it a lifestyle influence or a remnant of evolution?

To investigate sperm quality over time, researchers analyzed semen samples from 15,581 men aged 18 to 45 who applied to become sperm donors between 2018 and 2024. Samples were collected in four cities in Denmark (Aarhus, Aalborg, Odense, and Copenhagen) and in Orlando, Florida.

The researchers 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 in large circles, while non-gradually motile sperm move in narrow circles but not forward.)

Because it takes about 74 days for sperm to develop in the body, the researchers also looked at whether temperature in the weeks leading up to ejaculation affected sperm quality. However, they found little evidence that body temperature at the time of ejaculation or body temperature two months earlier had any measurable relationship with sperm quality.

However, the researchers hypothesized that temperature may affect sperm quality indirectly because it affects lifestyle factors known to influence sperm quality.

“The fact that seasonality is still present when considering ambient temperature leads us to think that other lifestyle changes may be important,” Pacey said. “This could include diet, exercise, and sun exposure. But we haven’t measured these things, so we can only speculate.”

Silver thinks this seasonal pattern may be a sign of evolution. Many animals living in temperate climates time their reproduction so that offspring are born in the spring, when conditions are better and resources are more abundant. If sperm quality peaks in the summer, the timing may increase your chances of having a baby in the spring. However, Silver suggested that humans have adapted to survive well in winter, so the effects of this season are likely weakening.

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Different studies, different trends

Previous studies have also reported seasonal changes in sperm quality, but the documented patterns were inconsistent. Several studies, including one in Italy, also found that sperm motility peaks during the summer, consistent with the new findings.

However, other studies have reported the opposite trend. For example, a large-scale analysis of more than 21,000 semen samples from southern China found that sperm motility peaks in late winter and declines during summer. Differences between these studies suggest that environmental factors such as regional lifestyles and differences in temperature, humidity and social behavior may shape how sperm quality fluctuates over time, study authors say in a new paper.

A new study found that in addition to seasonal variations, there is a strong association between sperm quality and age. Sperm motility was highest in men in their 30s and lowest in men under 25 and over 40.

The researchers also observed a significant decline in sperm quality in Denmark from 2019 to 2022, followed by a recovery in 2023, likely reflecting lifestyle changes during the COVID-19 pandemic. For example, lockdowns have changed the working patterns, diet, and physical activity levels of the general public, and previous research suggests that each of these factors can influence sperm motility.

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