Sometime in the summer of 1923, a forty-year-old French woman named Gabrielle returned from a holiday cruise in the French Riveria with a sunburn. I imagine this must have been a common occurrence, and surely would have gone unnoticed except for one thing: Gabrielle was actually known as Coco. As in Coco Chanel. The fashion icon’s newly bronzed skin became an instant trend among her fans that subsequently catalyzed a widespread obsession with sun-tanning.
No disrespect to the immense influence of Coco Chanel (apparently she is also credited with freeing women from the suffocating corset), but her sunburn was particularly timely. Not only was the Victorian-era aesthetic reverence for pale skin already fading, but sun exposure was just recently being heralded as the new ‘cure-all’ for a wide variety of diseases and illnesses. So it’s easy to understand the enthusiasm with which people embraced sun-tanning. Good for your health and fashionable - when does that ever happen?
Fast-forward almost a hundred years, however, and our understanding of sun exposure has changed drastically. UV light is a major risk for all types of skin cancer. Shockingly, one study estimated that a higher percentage of people develop skin cancer from repeated indoor tanning than lung cancer from smoking1. That is insane. Yet despite an increased awareness of the link between UV-exposure and skin cancer, sun-tanning persists as an immensely popular pastime for millions of Americans. Continuing to behave a certain way despite knowing there are negative consequences is a hallmark of addiction. Could the obsession with sun-tanning be more serious than an aesthetic adoration for tanned skin? Could it be that UV light somehow directly impinges on the brain’s reward system to induce a biological addiction akin to drugs of abuse?
In the last decade, more and more evidence suggests this is indeed the case. Many UV seekers - those who repeatedly and persistently suntan outdoors and in tanning salons - meet the standard diagnosis requirements for a substance-related disorder. And just like you can’t trick a cocaine addict with baking soda, you can’t trick a tanning addict with UV-free light - they can tell the difference2. What’s even more striking is that sun-tanning isn’t just analogous to substance abuse, it appears like it is substance abuse: specifically, opioid addiction. Opioids as in heroin and prescription painkillers. If given an opioid blocker during UV exposure, some tanning addicts go into withdrawal, suggesting that addiction to UV light is really an addiction to opioids3,4.
Evidently there are no opioid molecules hiding in UV light. So the most straightforward hypothesis is that UV light is somehow triggering the body to produce its own endogenous opioids which ultimately increase dopamine release in the brain to engender reward and addiction. Now figuring out if and how that actually happens is much harder to do in humans, but a recent study using mice showed that exposing mice to UV light (they actually shaved the fur off to maximize skin exposure) increased blood levels of beta-endorphin5. Beta-endorphin is an endogenous opioid most famous for its incredible capacity to act as an analgesic when released following a physical trauma (‘shock’) and for producing euphoric feelings during exercise (‘runner’s high’). It interacts with the same protein that morphine, heroin, and all opioid prescription painkillers bind - the mu-opioid receptor.
It turns out that when cells in the outermost layer of your skin (keratinocytes) are exposed to UV light, a signaling cascade is initiated that results in the production and secretion of beta-endorphin5. The beta-endorphin then diffuses into the blood where it circulates throughout the body, including the brain. Once it gets in the brain - well, then the rest of the story is familiar. Like other opioids that the brain produces, beta-endorphin is known to increase dopamine release to trigger feelings of reward and reinforce behaviors; in excess, this is exactly what causes addiction.
While all the evidence appears to support UV-induced beta-endorphin release as the cause of sun-tanning addiction, the case isn’t fully closed. No study has directly shown that mice repeatedly exposed to UV light will continue to seek it out in the face of negative consequences - a key feature when defining addiction in humans. Thus, it remains to be known whether people really become addicted to UV-induced beta-endorphin release, or if instead it’s just one component of UV-light addiction.
As with all studies done in model organisms, we must be cautious when applying the results found in mice to humans. In particular, one difference between mice and humans seems a little mystifying with respect to studying sun-exposure: mice are nocturnal. So it’s perhaps quite reasonable to postulate that mice evolved specific mechanisms to ensure enough sun exposure to be healthy (i.e. get enough vitamin D), and that humans would not need such a mechanism since we are awake during the day. It would be interesting to compare UV-induced beta-endorphin release in nocturnal and diurnal animals to see if the phenomenon is conserved.
Regardless of this wrinkle, I think it’s safe to strongly suspect UV-light as an addictive substance, and as such should be treated with the same cautiousness and respect as we do other substances of abuse. So while it might have been the celebrity sway of Coco Chanel that started an industry of sun-tanning, its continued persistence despite the discovery of UV light’s carcinogenic properties likely has something more to do with our evolutionarily ancient fondness for opioids rather than just cultural aesthetics.
- Wehner, M. R. et al. International prevalence of indoor tanning: a systematic review and meta-analysis. JAMA dermatology 150, 390–400 (2014).
- Feldman, S. R. et al. Ultraviolet exposure is a reinforcing stimulus in frequent indoor tanners. J. Am. Acad. Dermatol. 51, 45–51 (2004).
- Kaur, M., Liguori, A., Fleischer, A. B. & Feldman, S. R. Side effects of naltrexone observed in frequent tanners: could frequent tanners have ultraviolet-induced high opioid levels? J. Am. Acad. Dermatol. 52, 916 (2005).
- Kaur, M. et al. Induction of withdrawal-like symptoms in a small randomized, controlled trial of opioid blockade in frequent tanners. J. Am. Acad. Dermatol. 54, 709–11 (2006).
- Fell, G. L., Robinson, K. C., Mao, J., Woolf, C. J. & Fisher, D. E. Skin β-endorphin mediates addiction to UV light. Cell 157, 1527–34 (2014).