Can sunshine be as addictive as heroin?

sunbather on beach with bookIf recent media reports are to be believed, then sunshine can be “as addictive as heroin.” Although the study on which the reports were based was carried out on mice, the claims were surprising as sunlight does not typically fall into the list of common addictions such as cigarettes or alcohol. We looked at the media claims and found that the comparison was inaccurate, but ‘tanning addiction’ appears to have a scientific basis and several studies have investigated it.

True or False?

There is no doubt that sustained exposure to ultraviolet light can bring about (mainly psychological) symptoms similar to those that drug addicts experience, although they are nowhere near as extensive. Dr David Fisher who led the study behind the reports, questioned claims that sunshine is “as addictive as heroin” without analysis into the potency of heroin versus UV light. Dr Clare Stanford also told us that a mere ‘prefence‘ towards sunlight does not indicate addiction, and so it is largely false that sunshine can be as addictive as heroin.

Largely FalseAnalysis

Excess exposure to ultraviolet light, through sunbeds or natural sunlight, has become a significant concern in the past few years, due it its close association with skin cancer. Media reports emerged in mid June 2014 claiming that a new study proves “bathing in the sun has a similar effect on the human body as heroin”, with the International Business Times even claiming that “sunshine is as addictive as heroin.” The study, which appeared in the journal Cell, investigated the links between ultraviolet (UV) rays and the ‘opioid receptor pathway’ (the system which controls pain, reward and addiction). The results suggested that ultraviolet (UV) rays from the sun stimulated and elevated the production of endorphins in mice. These endorphins (known as ‘feel good’ hormones) are known to act on the same biological pathway as opioid drugs like heroin and morphine.

The experiment exposed a group of mice to artificial UV light for six weeks, followed by administration of naloxone, which blocks opioid receptor pathways. After just a week in the artificial sunlight, endorphin levels in the blood of the shaved mice increased. Swiftly denied the effects of UV light due to naloxone, the mice suffered a series of withdrawal symptoms such as shaking, teeth chattering and tremors. The UV exposure also caused the animal’s tails to stiffen and lift (known as ‘Straub tail’), an effect seen when mice are given opioid drugs such as morphine.

Concluding that chronic ultraviolet light exposure causes dependency and “addiction-like” behaviour, lead scientist Dr David Fisher suggested the findings may help to explain why people love sunbathing in addition to an aesthetic preference for tanned skin. Sunbathing may be the most natural method of satisfying our need for UV light, in the same way that drug addicts depend on their drug of choice.

Tanning addiction is a well recognised phenomenon and has similarities to other addictions such as drugs, alcohol and smoking. People with tanning addiction have a higher-than-average chance of also being affected by mood disturbance, obsessive-compulsive disorder and other psychiatric conditons. They often report mood improvement with the tanning process. However, many of the symptoms associated with drug addiction (e.g. behavioural changes, hyperactivity, anxiety, fatigue and nausea) are generally not observed in tanning addicts.

Academics had long suspected that sun seeking behaviour could fit the clinical criteria for a substance-related addiction, without knowing the underlying causes. In 2005, a study published in the Archives of Dermatology found that against two different criteria, between 25 and 50% of 145 beach-goers would qualify as having a substance-related disorder if UV light was classed as a substance. Frequent tanners were found to have experienced a “loss of control” over their tanning schedule and displayed a pattern of addiction similar to smokers and alcoholics.

A year later, a Wake Forest University study reported that frequent tanners (those who tanned 8-15 times a month) who took naltrexone (an endorphin blocker used to treat drug addictions) significantly reduced the amount of time spent tanning compared to a control group of light tanners.

Later in 2008, a study published in the American Journal of Health Behavior reported that 27% of 400 surveyed students were classified as “tanning dependent”. The authors claimed that those classed as being tanning dependent had a number of characteristics similar to drug addicts, including: a higher prevalence among youth; an initial perception that the behaviour was image enhancing; high health risks and disregard for warnings about those risks; and the activity being mood enhancing.

However, experts in the UK, most notably Dr Clare Stanford, urged caution when interpreting Dr Fisher’s research. She said: ”This study does not provide the sort of evidence needed to show addiction to UV light in mice and it is even less certain that the work predicts addiction in humans [which] would require testing whether the mice preferred UV light or non-UV light. The strain of mice used in this experiment produce virtually no melatonin, which is thought to protect against damage from UV light. Shaving such mice and exposing them to UV light raises important ethical questions about animal welfare and again casts doubt on the relevance of the results to humans.”

We contacted Dr Stanford to ask whether sunshine “stimulates the so-called ‘pleasure centre‘ in the brain and releases…endorphins” that bring about addiction. Describing the Telegraph headline as “ambitious”, she said her doubts were not due to concerns about extrapolation of the results from mice to humans but because she believed there were other potential explanations for the behavioural changes in the mice. Dr Stanford repeated that “testing would be required to show whether the mice preferred UV light or non-UV light, which was not done in this paper (to show that mice developed a strong preference for UV)”. Moreover, she believed a single test was insufficient evidence to conclude that UV light can be addictive, adding: “all things pleasurable stimulate [that ‘pleasure centre’], including food and water, but they are not all addictive.

Dr Fisher, the lead scientist behind the original study, told us the Telegraph headline “may be going far”, and added:

This study did not seek to prove that mice had become addicted to UV. The addictive nature of UV has previously been demonstrated in humans…using broadly accepted psychiatric indicators of addiction. This study instead focused upon understanding…[the basis] for that known response…the data from our paper suggest that UV…stimulates the same pathway which is the target of heroin, but it does not address potency comparisons [between heroin and UV light]. While heroin-like magnitudes of opiate pathway stimulation probably do not happen after typical sun exposure…more modest sun-seeking…may cumulatively produce enormous [eventual] consequences in terms of skin cancer incidence in the population (skin being the most common site for cancer in humans).

NHS Choices called the Daily Mail’s headline: “Sunbathing … is like heroin use”, “a little over-the top” and observed that “only very far into their coverage did the Mail reveal that the study was in mice.

Other scientists, such as Dr Richard Weller, senior lecturer in dermatology at the University of Edinburgh, explained that this preference for sunlight may be a survival mechanism that evolved over thousands of years because humans need vitamin D to survive.

Dr Weller stated: ”It is very unlikely that evolutionary pressures would select for a trait which reduces survival and reproductive ‘fitness’. If an ‘addiction’ to sun…exists in mankind, it suggests…there is a benefit to it.” He cited as evidence a large scale epidemiological study which showed that increased sun exposure is associated with reduced all-cause mortality.

In response to Dr Weller’s comments, Dr Fisher told us: “…I agree with Dr Weller’s suggestion that the presence of an addiction to sun…[implies] an evolutionary benefit to it at an earlier time, although in the modern era that benefit is likely countered by preventable cancer risks. Even all-cause mortality benefits can include preventable lethality and morbidity, and UV induced skin cancers are likely among those.


The evidence suggests there are compelling reasons to be concerned about the effect of UV light on humans, partly due to its positive psychological effect and importantly, due to the potential for it to cause chemical changes (increased endorphin production) in the body. Nevertheless, Dr Fisher was cautious about likening the effects of heroin to that of UV light, and said that comparisons between the impact of the two on the body would be incomplete without further analysis. He told The Henry Mayhew Foundation: “This information might serve as a valuable means of educating people to curb excessive sun exposure in order to limit skin cancer risk …. Our findings suggest that the decision to protect our skin…may require more of a conscious effort rather than a passive preference.” Dr Fisher also expressed his surprise that humans were seemingly “genetically programmed to become addicted to something as dangerous as UV radiation, [which is] probably the most common carcinogen in the world.

Dr Weller of the University of Edinburgh offered us a different perspective: “If the sun increases lifespan, even at the expense of skin cancers- why are we so against it?” While the media headlines were generally “ambitious”, the concern about UV light is well placed and because skin cancer is one of the most common forms of cancer in the world, research into the effects of sunlight on the human body is highly important.


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