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Spermatozoa: faster in summer than in winter? Here’s what a large study reveals

undefined undefined 3 mars 2026 undefined 20h00

The Editor

@ Dreamworks Pictures

Does the season affect male fertility? A new international study suggests that sperm may swim faster in summer than in winter. It's a subtle change but potentially significant for couples trying to conceive.

Does fertility follow the rhythm of the seasons? A study involving over 15,000 sperm samples sheds light on this seasonal phenomenon. Researchers from Denmark, the United Kingdom, and Canada have studied sperm donations from Danish and American men to assess the quality of sperm based on the time of year.

The scientists paid special attention to progressive motility, which refers to the ability of sperm to move effectively. It's these most mobile cells that have the best chances of fertilizing an egg. The result? The concentration of the fastest sperm increases in spring,

reaches a peak at the beginning of summer, then gradually decreases to reach its lowest level in the heart of winter. 


It's a matter of mobility, not quantity

Should we conclude that men are more fertile in summer? Not so fast. The study shows that the total sperm count and the volume of semen remain stable throughout the year.. The overall production doesn't change with the seasons. What does fluctuate, however, is the quality of their movement.

In other words, men don't become infertile in winter, nor do they suddenly become ultra-fertile in summer. However, the proportion of sperm that can swim effectively does seem to be influenced by the season.For couples trying to have a baby, this detail is important. A sperm analysis done in the summer might seem more favorable than a test done in winter, simply because of this natural variation.


Is temperature the culprit?

Scientists have known for a long time that the production and development of sperm is sensitive to temperature. In fact, the testicles operate at a temperature slightly lower than the rest of the body. However, earlier studies sometimes suggested the opposite: better sperm quality during the colder months. But these studies often involved small samples and yielded contradictory results.

By comparing a large number of samples from two countries with very different climates, this new research suggests that the seasonal pattern observed might be more consistent than previously thought. However, there’s a major limitation: it's an observational study. It highlights a trend, but can’t precisely explain the biological mechanisms at play, such as temperature, light exposure, lifestyle, or diet... all factors that could be involved.