Does season affect sperm quality? A large international study reveals a surprising summer peak
A major research study led by Cryos International reveals clear seasonal trends in sperm quality - and surprisingly, these trends appear not just in cold-weather countries like Denmark, but also in warm, sunny Florida.
The findings challenge the common assumption that temperature alone explains seasonal changes in male fertility and provide some of the strongest evidence to date that seasonality itself plays an important biological role.
What is the study about?
The study analysed over 15,000 semen samples from men who applied to become sperm donors between 2018 and 2024. Samples were collected from Cryos’ locations in four Danish cities and in Orlando, Florida, making it one of the largest and most detailed studies of its kind. All samples were analysed using the same lab systems and analytical standards.
This high level of consistency makes the research one of the largest and most methodologically robust studies ever conducted on seasonal variation in semen quality. Earlier studies have often produced conflicting results due to small sample sizes or differences in laboratory methods and climates. By overcoming these limitations, the new research provides unusually reliable insights.
The main results
The most striking finding was a strong and consistent seasonal pattern in the concentration of highly motile sperm (grade A sperm), being the fastest swimmers. In both Denmark and Florida, levels of these high-quality sperm varied significantly across the seasons, with a peak in early summer (May to July) and drop during winter (October to March). This pattern was consistent across locations, despite major differences in climate.
Notably, the concentration of grade A sperm increased by about 2 million sperm per mL from mid-winter to early summer. This represents a 31% increase in Denmark and a 13% increase in Florida, showing a clear and significant summer peak. This seasonal variation in the most motile sperm is a key finding, particularly as previous studies have often reported peak sperm motility during winter. Instead, this large-scale analysis shows a clear summer advantage.
Equally important is what did not change. The researchers found no seasonal variation in total sperm concentration or ejaculate volume. In other words, men appear to produce roughly the same number of sperm throughout the year, but the ability of those sperm to swim effectively improves significantly in summer. This distinction highlights that fertility is not only about quantity, but also about function.
The Florida results are especially interesting. While seasonal changes in Denmark were closely linked to outdoor temperature, the same summer peak was observed in Florida, even after accounting statistically for temperature differences. Because Florida remains warm year-round, this suggests that temperature alone cannot explain the seasonal variation. Other factors may be involved, such as changes in daylight exposure, vitamin D levels, lifestyle, diet, alcohol or tobacco use, sexual activity, or environmental exposures. The persistence of the pattern across two very different climates strengthens the conclusion that seasonality itself influences sperm motility.
The study’s future implications
These findings can have practical implications for male fertility care. Understanding seasonal patterns may help fertility clinics interpret semen analyses more accurately and optimise the timing of treatment. For sperm banks, the time of year when a donor is evaluated could influence whether he meets quality criteria. For couples trying to conceive, the findings suggest that male fertility potential may naturally fluctuate over the course of the year.
Overall, this large international study provides compelling new evidence that male reproductive biology follows a seasonal pattern, even in consistently warm climates. By demonstrating that sperm motility improves significantly in summer across different environments, the research improves our understanding of male fertility and opens the door to further investigation into the biological and lifestyle factors that drive these seasonal changes.