Sperm Works Best in the Winter, Study Finds

Spring may be when a young man’s fancy turns to love, but new evidence suggests that it’s winter when his sperm is at its spunkiest. 

Researchers at Israel’s Ben-Gurion University of the Negev found that sperm concentration and the percentage of fast motility—the ability to move spontaneously and independently—decreased significantly from spring into summer and fall, rebounding in the winter. (Find out how a man produces 1,500 sperm a second.)

Human sperm is seen in an microscopic image. Photograph by Dennis Kunkel, Visuals Unlimited


The physical structure of the sperm cells was also the healthiest in the winter months, according to the study, which tested 6,455 semen samples over the course of three years.

“This study was aimed to explore the possibility that changing weather is somehow related to the quality of sperm, a phenomenon well known from the animal world,” study leader Eliahu Levitas said in an email.

Warm Weather Bad for Sperm?

It’s possible that temperature could play a role in sperm health, just as changes in light have been found to cause sperm variation in some animals.

For instance, “it makes sense that we might see seasonal differences in sperm production, because we know that when the testicles get too hot they work less efficiently,” said Allan Pacey, senior lecturer in andrology—the study of male reproduction—at the University of Sheffield in the U.K., who was not involved in the new study. (Related: “Confirmed: Couch Potatoes Have Lower Sperm Counts.”)

Even so, said Levitas, “the real reasons for a reduced male fertility are in my opinion still [largely] a mystery. We are still searching for the factors that cause variations, even in the fertile male population.”

Fall Baby Booms Explained

Meanwhile, the findings offer a possible explanation for seasonal birth patterns, suggesting that the well-documented phenomenon of high fall birth rates could be tied to seasonal changes in sperm quality. (See “Sperm Tracked in 3D—A First.”)

The researchers cross-referenced their findings with five years of data of the birth registry at Israel’s Soroka University Medical Center. They found that birth rates were highest in the fall, which reflects increased conception rates in the previous winter.

baby picture
Aviva Zigdon holds her baby boy at the Soroka University Medical Center in Beersheba, Israel, in October 2011. Photograph by Ronen Zvulun, Reuters


“We were able to show the connection between the increase in sperm quality and the following increase in deliveries nine months later,” said Levitas, whose study was published in February in the American Journal of Obstetrics & Gynecology.

Conception Rx

While the findings will be of interest for couples trying to conceive, it doesn’t mean people shouldn’t give it the old college try throughout the year—and as often as possible. (Related: “Valentine’s Science—Why Gauging Sexiness Is Sophisticated.”)

“It would be inappropriate to suggest couples only try and conceive in certain seasons, as what is important is that couples have regular sex over the course of the year,” said Pacey.

Who would object to that prescription?


Stefan Sirucek is a writer and journalist who reports from both sides of the Atlantic. He's written for the Huffington Post and Wall Street Journal. Follow him on Twitter at @sirstefan.