Ultraviolet light exposure influences hormones and sexual behavior Original paper

Humans display a notable seasonal fluctuation in levels of hormones^[1] and birth rates^[2], with the former potentially influencing the latter due to its effects on sexual and romantic inclinations. One theory is that exposure to certain wavelengths of light influences the endocrine system in a manner that results in increased sex hormones. This study decided to test this theory.

This study used data from studies on both mice and humans.

For the mouse experiments, the investigators exposed the skin of shaved mice to either a low dose (50 millijoules per square centimeter; mJ/cm²) or an active dose (800 mJ/cm²) of ultraviolet B (UVB) light per day for 8 weeks. The active dose was comparable to 20 to 30 minutes of midday sun exposure and did not produce a sunburn.

The human studies employed several different designs and cohorts.

First, the investigators recruited 19 people (ages 23 to 73) with skin conditions who were undergoing phototherapy; they received a dose of 0.1 to 2.5 J/cm² twice or three times per week. Before the treatment and then 1 month after, investigators administered an adapted Passionate Love Scale (PLS) questionnaire, which assesses passion, attraction, and sexuality-related feelings about romantic partners. The investigators also administered a questionnaire evaluating aggression.

Next, the investigators recruited 19 people (ages 18 to 55) to avoid the sun for 2 days and then expose themselves to 25 minutes of midday sun (estimated ultraviolet light dose of 2,000 mJ/cm²). The investigators assessed levels of sex hormone markers both the day before and shortly after sun exposure.

Finally, the investigators used data on 13,086 men (ages 21 to 25) from a cohort study in Israel to assess average testosterone levels by month of the year. They also examined differences in testosterone levels by month among men from countries with low (<2,500 joules per square meter; J/m²) or high (≥4,500 J/m²) amounts of ultraviolet radiation (UVR), a proxy for skin color (which can influence the absorption of ultraviolet light).

First, the mouse data.

Male mice exposed to UVB light experienced increases in the sex hormones beta-estradiol, testosterone, and estrogen, and reduced anxiety. Female mice exposed to UVB light experienced increases in estrogen, androgen, beta-estradiol, and progesterone. Male mice were more attracted to UVB-exposed female mice, which were more receptive to sexual activity. These effects appeared to depend on the protein p53 in skin, which is involved in responding to DNA damage.

Next, the human data.

Men in the phototherapy group experienced increased obsessive thoughts about their loved ones, increased yearning to know everything about their partner, and an increased desire for affection from their partner, but their attraction to their partner decreased and verbal aggression increased. In the phototherapy group, the women’s feelings that their romantic partner is perfect increased, as did physical response to their touch.

In the sunlight exposure group, men experienced an increase in markers of the sex hormones beta-estradiol, progesterone, testosterone, and estrogen after sunlight exposure, and the women’s markers of estrogen, progesterone, and testosterone increased.

In the cohort study, men’s testosterone levels peaked in July. In summer, testosterone levels were higher among men from countries with low UVR compared with high UVR, but there was no difference in winter, which is consistent with an effect of ultraviolet light.