Of Mice and Men, Sun and Sex

Skin keeps all of our parts inside; add in sunlight’s UV rays, and it makes Vitamin D an essential nutrient. Oh yes, skin and those same UV rays enhance romantic passions in men and women – what’s up with that?

Before you go off to book that tanning bed (which can be dangerous), let me point out that the study has the details worked out more for mice than men, but it does report on humans.  

“It has been known for many years now that ultraviolet radiation from sunlight increases testosterone levels in males, and we also know that sunlight plays a major role in both the behavioral and hormonal regulation of sexuality.”

Carmit Levy, PhD

The study was initially carried out in mice exposed to the laboratory equivalent of 20 to 30 minutes of midday sun for a two-month period, enough to get “the juices flowing,” but not enough to cause what we would call “sunburn.” They studied mice courting behavior – grooming behavior, getting physically closer to their love interest, increased sniffing at their “nether regions,” and speed to the deed. They also studied their hormone levels and their effect on their reproductive organs.

  • There was a great attraction between female and male mice
  • “Both were more willing to engage in sexual intercourse.”
  • Induced hormonal changes that resulted in a greater period of sexual receptivity in the females as well as increasing the size of their ovaries [1]

Guardian of the Genome

Let me introduce p53, a protein known by that nickname. It is involved in regulating uncontrolled cell growth and division; in the skin, it is involved in preventing sun damage that would result in cancers. In a series of secondary mice experiments, we will get to the humans in a moment, the researchers knocked out the gene responsible for p53. The effects of sunlight exposure on mice behavior and physiology disappeared. This was the key take-away for scientists, that p53 contributed to a skin-brain-gonadal sexual axis. 

But enough about mice, what about us!

The researchers recruited 32 patients undergoing medically prescribed phototherapy, all receiving a known dose of UVB exposure. The participants completed the Passionate Love Scale questionnaire before beginning treatment and one month after beginning 10-20 minutes, 2-3 times a week phototherapy.

Males developed more obsessive thoughts about their “loved one, “yearning to know everything about her, and endless desire for affection from her.”

Females began to feel “that the person they loved most passionately is the perfect romantic partner and experiencing a physical response when touched by that person.”

Testosterone increase from sunlight exposure in our male mouse buddies, the researchers found that male humans were also more “verbally” aggressive after phototherapy. 

A smaller group of participants were asked to avoid sun exposure for two days and then expose themselves to 25 minutes of bright midday sun. The researchers found increased hormonal levels of reproductive hormones in both men and women after sun exposure.

Fun “Facts”

We are all acquainted with the seasonality of plants. A similar cycle is seen in humans – a spring, or spring and fall “peak in conceptions.” Both sun exposure and temperature drive the variations. It is intriguing to consider how our exposure to sunlight has changed over time. The amplitude of the peaks has been reduced in parallel with industrialization as we move from outside to inside work. 

Vitamin D is a product of our skin and sunlight. It is not a perfect stand-in for sun exposure because you can find Vitamin D in tablets and milk. Before the pandemic, the millennials were noted to be having less sex. There are also studies documenting Vitamin D deficiencies in this age group and increasing concerns about human fertility. Could it be that we are just not getting as much sunlight as we need? Could the perfect date be walking in the park or the beach rather than dinner and a movie?

[1] Most females are most receptive to sexual advances during ovulation, the peak of their fertility in their estrus cycle. Human females are the one known group that is receptive throughout their cycle, although some husbands claim marriage ends that effect.


Source: Skin exposure to UVB light induces a skin-brain-gonad axis and sexual behavior Cell Reports DOI: 10.1016/j.celrep.2021.109579