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Published Research

Do changes in day length affect reproduction?

woman sat on the floor with her face towards the sunlight
  • Longer, lighter days were linked to a higher ovulation rate
  • During months with longer days, Natural Cycles users logged higher libido and had more sex

In the animal kingdom, reproduction is often closely linked to the seasons. Many animals time their mating so that they give birth in the spring or summer months when the weather is better and there’s more food available. This makes a lot of sense as timing reproduction when the weather is best means offspring have the best chance of survival.

Because we human beings are able to control our own habitat, we’re not as affected by seasonal changes. In addition, we also have access to birth control which gives us further power over our reproduction. However, previous research has indicated that human reproduction might still be affected by seasonal changes, with conception rates being higher in the spring and summer.

In this study, the Natural Cycles Research Team collaborated with external researchers led by Dr. Sarah Hill to investigate this topic further, looking into if summer days with increased daylight hours lead to increased ovulation rates and sexual behavior. 

To answer this question, the research project analyzed data from 76,180 Natural Cycles users who were located in the US and Sweden – a total of 425,909 menstrual cycles. We looked at the frequency of ovulation, logged libido and logged sexual activity, and analyzed if these were affected by different day lengths throughout the year.

The results showed that days with an increased number of daylight hours were in fact linked to higher ovulation rates, meaning that ovulation was detected in more cycles during the months with longer days. This finding was present in both the US and Swedish groups, showing that it is valid across different locations, and the relationship between day length and ovulation rate was stronger when looking specifically at those in the age range that’s most likely to be fertile (18-45). 

On top of this, both groups saw an increase in sexual activity that corresponded to the increase in day length. In the US cohort, there was also an increase in libido levels being logged.

It’s possible that this last finding can be explained by physiological changes (like the increased ovulation rate), but it could also be related to non-physiological changes, for example, that people often go on vacation or have more free time during the summer months.

In summary, this study presents some of the first evidence that ovulation rates in humans vary with the seasons, and it also shows that both libido and sexual activity may follow a similar pattern. Although us humans are able to take control of our reproduction by using birth control or by actively planning a pregnancy, the results of this study are interesting since they may help explain how our behavior and feelings are influenced by the seasons. 

Not only does this kind of research add to a growing number of studies carried out in this historically under-researched field, but it also offers us the chance to learn and reflect more on our reproductive health as individuals.