A large-scale international study has revealed that men's sperm move more quickly during summer months compared to winter, according to research published in February 2026. Researchers from Denmark, the United Kingdom, and Canada analyzed more than 15,000 semen samples from donors in Denmark and the United States. The study found clear seasonal changes in semen quality, specifically in the concentration and number of progressively motile sperm - those that swim forward effectively and are most likely to fertilize an egg.
The concentration of the fastest-moving sperm rose through spring, peaked in early summer, and then fell to its lowest levels in mid-winter. The findings could have implications for couples trying to conceive and for fertility clinics interpreting test results, as a semen analysis taken in summer may look different from one taken in winter. However, researchers noted that the differences do not mean men are fertile only in summer or infertile in winter.
Overall sperm production did not change across the year, with total sperm count and semen volume remaining stable in all seasons. What varied was how well the sperm moved. The study was observational, meaning it shows a pattern but cannot explain exactly why it happens, though scientists have long known that sperm development is sensitive to temperature.