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By Daniella Gray
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If you’re trying to conceive, timing may matter in more ways than previously thought.
According to a large new study, men tend to produce their highest‑quality sperm during the summer months, with fertility dipping in winter.
Researchers at the University of Manchester, Queen’s University in Ontario, Canada and Cryos International in Aarhus, Denmark who analyzed semen samples from more than 15,000 men found that sperm motility—how well the cells can swim—consistently peaked in June and July and was lowest in December and January.
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The samples were collected from men aged 18 to 45 who applied to be sperm donors between 2018 and 2024.
Participants lived near clinics in two very different climates: Denmark and Florida. Despite major differences in weather, researchers found nearly identical seasonal trends in both locations, suggesting the changes were not driven by temperature alone.
What changed with the seasons was not the number of sperm men produced, but how effectively those sperm could move.
Fast‑moving, progressively motile sperm—the type most capable of reaching and fertilizing an egg—were most abundant in early summer and scarcest in mid‑winter.
Meanwhile, total sperm count and semen volume stayed relatively stable throughout the year.
“We were struck by how similar the seasonal pattern was in two completely different climates,” paper author professor Allan Pacey of the University of Manchester said in a statement.
He added: “Even in Florida, where temperatures stay warm, sperm motility still peaked in summer and dipped in winter, which tells us that ambient temperature alone is unlikely to explain these changes.”
Scientists have long suspected that sperm quality might fluctuate seasonally, but past studies often produced conflicting results due to small sample sizes or inconsistent testing methods.
This research stands out because all samples were analyzed within an hour of collection using the same standardized equipment, reducing variability and strengthening confidence in the findings.
The team also examined outdoor temperatures during the month of collection and in the two months prior—a critical window when sperm develop. Even after accounting for heat exposure, the seasonal pattern remained.
That suggests other factors tied to the seasons may play a role, such as daylight exposure, sleep patterns, physical activity, diet, stress or seasonal illnesses.
The research team said the findings could have practical implications for fertility clinics and couples trying to conceive. Understanding when sperm quality tends to peak could help guide the timing of fertility testing or treatment, though researchers caution that conception is influenced by many factors and can happen at any time of year.
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Reference
Lassen, E., Pacey, A. A., Skytte, A.-B., & Montgomerie, R. (2026). Seasonal trends in sperm quality in Denmark and Florida. Reproductive Biology and Endocrinology. https://doi.org/10.1186/s12958-026-01537-w
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