Sex Ed

Exposure to Sunlight May Boost Testosterone and Passion

October 27, 2021 by Justin Lehmiller

If you’re a long-time reader of the blog, you’ve probably seen articles I’ve posted about seasonal variations in sexual behavior. According to the research, one of the biggest peaks occurs during the summer months. For example, people report having more partners during the warmest months and, at the same time, rates of sexually transmitted infections tend to rise.

So why is that? It’s often attributed to the fact that people tend to take more vacations in the summer, which leaves people with more leisure time. However, new research suggests there might be something else to it—in fact, it might have something to do with greater exposure to sunlight.

In a set of studies published in the journal Cell Reports, researchers found that exposure to UVB light under controlled conditions is linked to changes in hormones and sexual behavior in animals and humans alike.

The research started with two studies of rodents. In the first, they found that exposing mice to UVB light had significant effects on their bodies and behavior. For example, female mice spent more time in heat (i.e., they had more estrus days), which corresponded with a rise in hormones that govern their reproductive cycle. Further, the males experienced an increase in plasma testosterone—and more mating behavior occurred across sexes.

Next, they genetically modified a group of mice to remove a skin protein that responds to sunlight and leads to skin pigmentation. For these mice, exposure to UVB did not lead to the same changes in hormones and sexual behavior that were previously observed. In other words, when their bodies were no longer registering sunlight exposure in the same way, the effect disappeared.

Finally, they studied whether something similar happens in humans. In one variation of the study, male and female participants were asked to avoid sunlight for two days and then spend 25 minutes in the sun. Comparing blood samples from a day without sunlight to post-sunlight exposure, the researchers documented a rise in testosterone as well as several other sex hormones in men and women alike.

They also looked at data from patients undergoing phototherapy who were asked to fill out a scale measuring feelings of passionate love both prior to their first UVB treatment and again after 10-12 treatments. What they found was that, for men and women alike, feelings of passionate love increased following treatment. Also, for men only, there was a rise in verbal aggression after treatment.

They also analyzed existing medical data from thousands of men who had their hormone levels tested throughout the year, which revealed a rise in testosterone during the summer months (specifically, there was a peak in July-August compared to the rest of the year).

What all of these findings suggest is that exposure to sunlight appears to affect hormone production and sexual interest/behavior, and this appears to be mediated through proteins on the skin that respond to sunlight. Specifically, the proteins that exist to protect our skin (and that give us a suntan) may also trigger changes in our sexuality.

As for why this effect exists, we can’t say. Did humans and animals evolve to mate and reproduce more during better weather seasons? Or do we just experience these effects more in the summer than the winter because the UV index is higher, we’re usually spending more time outdoors, and wearing less clothing?

That said, this research does have intriguing implications. Not only does it help us to better understand seasonal fluctuations in sexual interest and behavior, but it also suggests a potential future treatment possibility for persons with hormonal imbalances. Of course, more research is needed before we can recommend this, but UVB light just might have therapeutic value when it comes to our sex lives.

To learn more about this research, see: Parikh, R., Sorek, E., Parikh, S., Michael, K., Bikovski, L., Tshori, S., … & Levy, C. (2021). Skin exposure to UVB light induces a skin-brain-gonad axis and sexual behavior. Cell Reports36(8), 109579.

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Image Source: Photo by Frank Mckenna on Unsplash

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Written by
Dr. Justin Lehmiller
Founder & Owner of Sex and Psychology

Dr. Justin Lehmiller is a social psychologist and Research Fellow at The Kinsey Institute. He runs the Sex and Psychology blog and podcast and is author of the popular book Tell Me What You Want. Dr. Lehmiller is an award-winning educator, and a prolific researcher who has published more than 50 academic works.

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