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Major study finds a surprising link between season and sperm quality

Major study finds a surprising link between season and sperm quality

Men’s sperm moved fastest in summer and slowest in winter, according to research involving 15,000 men.

Published: February 25, 2026 at 10:13 PM

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Couples trying to conceive are often told that timing is everything. Now, new research suggests the time of year could also make a difference.

A major international study has found that sperm quality, specifically the number of actively moving sperm, is highest in the summer months and lowest in winter. The findings were consistent across men living in both Denmark and Florida, despite their very different climates.

Published in Reproductive Biology and Endocrinology, the study analysed semen samples from more than 15,000 men between 2018 and 2024.

What did the study look at?

Researchers analysed data from 15,581 men aged 18 to 45 who applied to be sperm donors at Cryos International between 2018 and 2024.

Of these, 10,670 were based in Denmark and 4,911 in Orlando, Florida. All samples were assessed using the same computer assisted semen analysis system and standardised laboratory protocols to ensure consistency.

The team focused on progressively motile sperm, sperm that actively swim forward and are considered crucial for natural conception.

Sperm motility peaks in summer

The strongest and most consistent pattern was seen in rapidly progressive sperm, the fastest swimmers.

In both Denmark and Florida, concentrations were highest in June and July and lowest in December and January.

There was no evidence of seasonal variation in overall sperm concentration or ejaculate volume. This suggests that sperm production itself does not fluctuate across the year, but the quality of movement does.

Total Motile Sperm Count, a key measure used in fertility assessments, also showed seasonal variation, with higher values in late spring and early summer and lower levels in winter.

Does temperature explain it?

Because sperm production is known to be temperature sensitive, the researchers examined whether outdoor temperatures influenced the results.

They modelled average monthly temperatures at the time of sample collection and two months earlier, when spermatogenesis begins.

While there was some evidence of a link between warmer temperatures and higher levels of rapidly progressive sperm, the seasonal pattern remained even after accounting for temperature.

The authors concluded that semen quality “exhibits clear and consistent seasonal variation” in both temperate and subtropical climates.

Semen quality (measured as the concentration and number of progressively motile sperm) exhibits clear and consistent seasonal variation in the male populations living in both a temperate and a subtropical climate.

Lassen E., Pacey A.A., Skytte A.-B. et al., Reproductive Biology and Endocrinology

Even when controlling for temperature, the seasonal patterns persisted, suggesting other factors, possibly linked to lifestyle, daylight hours or broader environmental influences, may also play a role.

What does this mean for families?

For couples navigating fertility challenges, the findings are a reminder that sperm quality is not fixed.

The researchers highlighted the importance of accounting for seasonality when assessing semen parameters for fertility evaluation.

That does not mean couples should delay trying for a baby until summer. Many factors influence fertility, including age, overall health and timing within the menstrual cycle.

The study also found sperm motility was highest in men aged between 25 and 35 in both locations , with lower levels seen in men under 25 and over 40.

Importantly, while seasonal patterns were clear at population level, there was considerable variation between individuals.

For families hoping to grow, the key takeaway is that healthy lifestyle habits, including maintaining a healthy weight, avoiding smoking and limiting alcohol, remain central to supporting fertility all year round.