Sunlight 'Enhances Passionate Love' in Humans, Study Suggests

Scientists have said that exposure to sunlight—specifically what are known as UVB rays—may boost romantic passion in humans.

The study published in the journal Cell Reports, which involved a small group of men and women who were exposed to UVB rays, found that both scored more highly on passionate love questionnaires after exposure than they did before exposure.

It also involved animal testing of mice which found increased levels of sex steroid signaling after UVB exposure.

The researchers believe the results are linked to hormone production and think they could lead to practical applications such as treating sexual hormone disorders with UVB treatments.

It should be noted that ultraviolet rays—and UVB rays in particular—can be dangerous, increase the risk of skin cancer, and can cause skin damage. A small amount of sunlight is healthy, but too much is dangerous and measures should be taken to reduce it, according to Johns Hopkins Medicine.

Professor Carmit Levy of the Department of Human Molecular Genetics & Biochemistry at Tel Aviv University, who co-authored the study, said in a press release: "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.

"However, the mechanism responsible for this regulation remained unknown. Our study enabled a better understanding of this mechanism."

As part of the study, the researchers examined both human volunteers and lab mice to see how UVB exposure affected sexual responses.

The human cohort involved 19 people in total—nine men and 10 women—who were recruited from two Israeli hospitals and were due to undergo UVB exposure.

They were asked to fill in a questionnaire developed to measure passionate love in intimate relationships before their first UVB treatment and then again one month afterwards. The treatment involved a UVB dose two or three times a week.

Questionnaires taken after UVB exposure showed that the males' scores in regards to a female parter were significantly higher than before "with respect to obsessive thoughts regarding their loved ones, yearning to know everything about her, and endless desire for affection from her," the authors wrote. However, they also reported "significantly less attraction" to that person.

Women, meanwhile, scored much higher in terms of "feeling that the person whom they loved most passionately is the perfect romantic partner and experiencing a physical response when touched by that person."

The results also indicated an increase in verbal—but not physical—aggression in men that was not present in women, suggesting a link to testosterone which men generally have higher levels of.

The researchers also looked at existing data on testosterone levels in around 13,000 men aged between 21 and 25 and found a peak in total testosterone during the summer.

The study concluded that the data suggests "UVB treatment enhances passionate love" in humans.

The researchers also evaluated the effect of UVB exposure on mice based on blood and ovary samples and behavioral observations.

For example, the researchers found that male mice were more vocal around female mice that had been exposed to UVB rays than those that had not been exposed to UVB rays. They also found that sex steroid signaling in mice increased after chronic UVB exposure.