Everyone must know by now to minimize their exposure to the sun, especially during the peak mid-morning to mid-afternoon hours, to reduce the risk of skin cancer from ultraviolet rays.

But Tel Aviv University (TAU) researchers have found that being in the sun can not only promote sexual attraction in rodents; when tested in humans, sunlight was found to enhance romantic passion in both men and women.

The study was led by doctoral student Roma Parikh and Ashchar Sorek in the lab of Prof. Carmit Levy at the department of human molecular genetics and biochemistry at TAU’s Sackler Faculty of Medicine. 

The sun produces three kinds of ultraviolet rays, UVA, UVB and the lesser-known UVC. UVA rays have slightly more energy than UVA rays and can damage the DNA in skin cells directly. They are thus  the main rays that cause sunburns and are believed to cause most skin cancers. UVC rays have more energy than the other types of UV rays.

People were willingly exposed under controlled conditions to UVB phototherapy (to treat certain medical problems) at the Tel Aviv Sourasky (Ichilov) and Assuta Medical Centers. 

The findings were clear and unequivocal – increased levels of romantic passion was measured in both genders. The study revealed that exposure to sunlight affects the regulation of the endocrine system responsible for the release of sexual hormones in humans.

The groundbreaking discovery was published as a cover story in the prestigious scientific journal Cell Reports under the title “UVB exposure increases circulating sex-steroid levels in mice and 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,” said Levy. “However, the mechanism responsible for this regulation remained unknown. Our study enabled a better understanding of it.”

The study began in an animal model, exposing mice to UVB rays at wavelengths of 320-400 nanometers. The effect was dramatic – the sex hormone levels in females rose significantly, enlarging their ovaries and prolonging their rut (breeding) season; the attraction between males and females increased and both were more willing to engage in sexual intercourse.

In the second stage, the researchers repeated the experiment in the animal model, this time removing from the skin a protein called p53, which identifies DNA damage and activates pigmentation during exposure to sunlight, as protection against its adverse effects. The removal of p53 eliminated the effect of UVB exposure on the animals’ sexual behavior, persuading Levy and her colleagues that exposure to radiation through the skin was the cause of the observed hormonal, physiological and behavioral changes and that the protective system is also responsible for the regulation of sexuality.

The last stage of the study included 32 people who filled out validated questionnaires on behaviors of romantic passion and aggression. Treated with UVB phototherapy in the hospitals, the patients were asked to fill an adapted Passionate Love Scale (PLS) questionnaire developed by researchers in 1986. 

The nine men and 10 women aged 18 to 55 years were asked to avoid sun exposure for two days and then spend 25 min in the sun on a bright sunny midday. This resulted in a dose of approximately 2,000 mJ/cm2 UV radiation (measured using a UVX radiometer). Blood samples were collected on the day before sun exposure and approximately the same time on the day of sun exposure. 

Similar to the mouse data, they found a significant positive activation score for β-estradiol, progesterone, testosterone and estrogen in men and for estrogen, progesterone and testosterone in women following solar exposure compared with the control day before exposure. 

The result was that both genders exhibited a rise in romantic passion, and males also noted an increase in levels of aggression.

Similar results were found when the subjects were asked to avoid sunlight for two days and then tan themselves for about 25 minutes. Blood tests revealed that exposure to sunlight resulted in a higher release of hormones like testosterone compared to one day before exposure. A rise in testosterone in males during the summer was also found in analyses of data from two Israel health maintenance organizations, Clalit and Maccabi Health Services.

“The skin contains various mechanisms for dealing with radiation from sunlight, and one of these is the p53 protein,” explained Levy. “We must remember that exposure to UV is dangerous, and can damage the DNA, as in the case of skin cancer. At the same time, two built-in programs in the skin, activated following exposure to sunlight are in place to protect against DNA damage – the DNA repair system and pigmentation (suntan) based on degree of exposure. By activating both systems, the p53 protein regulates the level of DNA damage. In our study we found that the same system also activates the endocrine system of sexuality and potentially breeding.”

Passion, according to the researchers, “takes two forms, emotional and sexual. UVB radiation affected different components of passion in men than in women. UVB-treated women scored higher on questions about physical arousal that related more to sexual passion and idealizing the connection, whereas men scored higher on the cognitive dimension of passion, which involves obsessive thoughts about the partner and wanting to know more about her. The questionnaire we used measured romantic passion, rather than physiological/sexual passion, due to institutional review board ethical concerns regarding sensitive sexually oriented questions. Future studies on this topic should address physiological arousal more directly and should be geared toward the precise identification of the different effects of UVB on the sexual behavior of men and women.”

Conception must be timed so that offspring are born when they have the highest chances of survival and reproduction. This is likely the reason for seasonality in birth rates in humans. There is a spring-summer (end of April to May) peak in conceptions in most of Europe and a strongly bimodal distribution in North America, with peaks in spring and autumn.  

Therefore, the researchers wrote, UVB may serve as a backup mechanism, ensuring optimal reproduction timing and direct influence on fitness. It is interesting that industrialization, which shifted work from outdoors to indoors with eternal summer conditions, characterized by a long photoperiod and mild temperatures with almost no seasonality, happened in parallel to an amplitude reduction in peaks in human conceptions observed in today’s industrialized nations. “It is possible that the reduced exposure to UVB contributed to this change.”

Sensory signals from the skin to the brain include temperature, touch, pain, stretch, itch, and vibration; they are sensed by skin receptors that transfer the stimuli via nerve fibers directly to the brain, they continued. 

Another mode of skin-brain crosstalk involves the combined neural signals from the preoptic hypothalamus in the brain and peripheral nerves that together trigger eccrine sweat glands. Although the neural triggers for pheromone synthesis and secretion are poorly understood, it has been established that eccrine sweat glands are controlled by hypothalamus cues. “Therefore, we cannot exclude the possibility that part of the observed effect is mediated by pheromones: UVB radiation may alter hypothalamus activity or directly affect axillary secretion. Both possibilities should be further investigated.”

In the future, this new discovery at TAU may lead to practical applications such as UVB treatments for sexual hormone disorders. But more research is still required before this can be achieved. According to Levy, the breakthrough will also lead to further discoveries in basic science. “Our findings open many scientific and philosophical questions. As humans, we have no fur, and our skin is then directly exposed to sunlight. We are only beginning to understand what this exposure does to us and the key roles it might play in various physiological and behavioral processes. It’s only the tip of the iceberg.”