A new scientific study suggests that human sperm quality follows a seasonal pattern, peaking during the summer months and declining in winter.
Published in the journal Reproductive Biology and Endocrinology, the study analyzed more than 15,000 semen samples from prospective donors in Denmark and Florida, USA.
Researchers found that levels of progressively motile sperm —those capable of swimming efficiently in a straight line— were highest in June and July, while the lowest levels were recorded in December and January.
“Seeing this pattern, we wondered whether men might be more likely to be accepted as sperm donors if they apply in the summer,” said co-author Allan Pacey, professor of andrology at the University of Manchester. He noted, however, that the idea that couples may achieve better conception outcomes in summer remains a hypothesis rather than a confirmed conclusion.
The indirect role of temperature
The study included 15,581 men aged 18 to 45 who applied as sperm donors between 2018 and 2024. Samples were collected in four Danish cities —Aarhus, Aalborg, Odense, and Copenhagen— as well as in Orlando, Florida.
Using advanced analytical methods, researchers measured semen volume, sperm concentration, and motility. Given that sperm development takes approximately 74 days, they also examined whether temperatures in the weeks prior to ejaculation had any effect.
No strong evidence was found linking temperature —either at the time of ejaculation or roughly two months earlier— to measurable differences in sperm quality. However, the researchers suggest temperature may still play an indirect role by influencing lifestyle factors such as diet, exercise, and sunlight exposure.
Conflicting findings and regional differences
Previous studies have also identified seasonal variation, though not always with the same pattern. Research in Italy has similarly reported a summer peak, while a large-scale study in southern China found the opposite trend, with higher sperm quality in late winter.
These discrepancies suggest that local environmental and social factors may shape seasonal reproductive patterns differently across regions.
The study also found a clear link with age: sperm motility was highest in men around 30 years old and lower in those under 25 or over 40.
In addition, Denmark saw a noticeable decline in sperm quality between 2019 and 2022, followed by a recovery in 2023 — possibly linked to lifestyle changes during the pandemic. In contrast, Orlando showed a steady increase from 2018 to 2024, a trend that remains unexplained.