Romantic getaways, outdoor concerts, heat exhaustion—and now, apparently, optimal sperm quality. Summer really does have everything.

A new study published in the journal Reproductive Biology and Endocrinology found that human sperm quality follows a seasonal pattern, peaking in June and July and bottoming out in December and January. Researchers analyzed semen samples from more than 15,500 men between the ages of 18 and 45 who applied to be sperm donors between 2018 and 2024, with samples collected across four Danish cities and in Orlando, Florida.

What they were measuring specifically was “progressive motility”—essentially, how well sperm swim in a straight line. The summer months won consistently across both populations.

Study co-author Allan Pacey, a professor of andrology at the University of Manchester, told Live Science the findings made him wonder whether aspiring sperm donors might have better odds of acceptance if they applied in warmer months. “By extension,” he said, “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.”

This Is the Weirdly Exact Time Your Sperm Quality Peaks

Before anyone starts rearranging their conception calendar, it’s worth hearing from Dr. Sherman Silber, a urologist and director of The Silber Infertility Center of St. Louis, who had no involvement in the study. He told Live Science the seasonal differences are “very, very tiny” and “make no difference whatsoever biologically.” So, temper your expectations accordingly.

What’s truly interesting here is that temperature alone doesn’t explain the trend. Because sperm take roughly 74 days to develop, the researchers looked at whether ambient temperatures in the weeks before ejaculation had any effect. They didn’t find a meaningful connection. Pacey said that lifestyle factors are likely doing lots of the work. “This could include diet, exercise, exposure to sunlight,” he said. “But we did not measure these things, so we can only speculate.”

Silber offered another theory: evolutionary baggage. In many animals, reproduction is timed so offspring arrive in spring, when survival conditions are better. A summer sperm spike would support that timing. 

The data also surfaced a few other notable findings. Sperm motility was highest in men in their 30s and declined in men under 25 and over 40. In Denmark, quality dipped noticeably between 2019 and 2022, which the researchers attributed to pandemic-era lifestyle disruptions, then rebounded in 2023. Orlando, meanwhile, saw a steady improvement from 2018 to 2024 for reasons nobody can fully explain yet.

Science, as always, answers one question and creates about six more.