SEPTEMBER 22nd marks the autumn equinox and, in the northern hemisphere at least, heralds the gloomy six-month period during which the nights will be longer the days. As a result, millions of sun-starved northern Europeans will flee to the beaches of the Caribbean or north Africa in search of some winter rays.
Their doctors would probably rather they stayed home. Besides ageing the skin prematurely, the ultraviolet (UV) radiation in sunlight also scrambles DNA. That causes skin cancer, worldwide rates of which are rising steadily. And although some sunlight is necessary to make vitamin D, this nutrient can also be obtained from food or pills. For that reason, public-health advice over the past few decades has tended to emphasise avoiding the sun, via seeking shade, covering up and using sun cream.
But perhaps that advice has gone a bit too far—at least for denizens of gloomy countries at high latitudes. A growing body of research hints at health benefits from sunlight that go beyond just those offered by vitamin D. These include protections against heart disease, cancer and autoimmune diseases. A study published last year, for instance, examined medical data from 360,000 light-skinned Brits and found that greater exposure to UV radiation—either from living in Britain’s sunnier southern bits rather than the darker north, or from regularly using sunbeds—was correlated with either a 12% and 15% lower risk, respectively, of dying, even when the raised risk of skin cancer was taken into account.
That fits with the results of another big study published a decade earlier. Led by Pelle Lindqvist, an epidemiologist at the Karolinska Institute in Stockholm, it followed 30,000 Swedish women for 20 years. It likewise found that, even after correcting for things like age, wealth and health, sun-seeking behaviour was associated with a lower chance of death from all causes. People with the most sun exposure had only half the risk of dying compared with those who had the least exposure.
“The big picture is that the benefits of sunlight outweigh the risks—provided you don’t get sunburnt,” argues Richard Weller, a dermatologist at the University of Edinburgh and one of the authors of the British study. Drs Lindqvist and Weller are two of the 17 scientists who also wrote a review paper, published in June, which urged public-health bodies to pay more attention to the growing evidence for the beneficial effects of UV radiation.
Evolution strongly suggests that sunlight has upsides. In sun-soaked Africa, where hairless humans first evolved, it equipped them with plenty of melanin, a pigment that helps protect skin from the DNA-scrambling effects of UV light. But once some of those humans had migrated north to places with weaker sun, the melanin levels of their descendants dropped, allowing in more UV radiation.
Light skin has evolved at least twice—once among modern Europeans and again among east Asians—and the evolutionary pressure that drove it is among the strongest seen in the human genome, says David Whiteman, a skin-cancer specialist at the QIMR Berghofer Medical Research Institute in Brisbane. It all suggests that UV radiation serves a purpose in human biology—and that both too little and too much sunlight is undesirable.
One of the upsides of sunlight is well known. UV radiation is necessary for the body to make vitamin D, a lack of which can cause soft bones and skeletal deformities in children. Higher levels of vitamin D in the blood are associated with all sorts of potential health benefits, from better heart health to lower cancer risk. But several big studies into the effects of vitamin D supplements have had disappointing results, says Amaya Virós, a skin-cancer researcher at the University of Manchester. (In 2022 an editorial in the New England Journal of Medicine argued that, given those findings, doctors should stop recommending their patients take them for general use.)
Some researchers now think that the association between vitamin D and good health is, in fact, explained by other chemical pathways influenced by the sun’s rays. There is no shortage of candidates: sunlight seems to affect the expression of many different genes. One mechanism that has attracted particular attention involves nitric oxide, a signalling molecule that, among other things, relaxes blood vessels and lowers blood pressure. In 2009 a group of researchers based in Germany showed that UV irradiation converts chemicals in the skin into nitric oxide, which then makes its way into the blood—and that whole-body exposure caused a quick and substantial drop in blood pressure.
That throws suggestive light on existing data showing that blood pressure tends to rise the farther from the equator you go. One 2017 paper found an increase of roughly 5mm of mercury (the units in which blood pressure is measured) for every thousand kilometres north of the equator. High blood pressure is a risk factor for heart disease—and death rates from heart disease in high-latitude countries also show a striking seasonal pattern, being highest in winter and lowest in summer (see chart). Some of that is down to colder weather, changes in diet and the like. But some researchers wonder if at least some of it could be down to lack of sun, too.
Another intriguing line of evidence concerns UV radiation’s effects on the immune system. For multiple sclerosis (MS), it appears to offer relief (like high blood pressure, MS seems to be more common in higher latitudes). Some scientists are exploring whether UV’s effects on the immune system might also improve its ability to combat cancers. In work that is currently unpublished, Drs Virós and Weller have studied a specific immune mechanism (unrelated to vitamin D) in both people and lab mice that may reduce the risk of cancer spreading to other parts of the body.
Other studies report an association between sun exposure and lower rates of diabetes (with evidence from mice once again implicating nitric oxide). A lack of sunlight is thought to be an important reason why, in some Asian cities, more than 80% of teenagers now need glasses. Bright light—of the sort that is hard to generate indoors—appears vital to regulate the growth of children’s eyes.
All this is fuelling calls by some researchers to tweak public-health guidelines to tone down the emphasis on avoiding the sun, and to acknowledge that the risks and benefits will differ between those with lighter or darker skin. “You can’t tell someone of African extraction he has the same risks and benefits from sun exposure as someone from a Scottish background,” says Dr Virós.In Britain, cardiovascular disease kills far more people every year (around 170,000) than skin cancer, which kills around 3,000, points out Dr Weller.
Not everyone is quite so gung-ho.Much of the new research pointing to benefits has been carried out only on light-skinned people in sun-starved countries, for one thing.Thepositiveresults on blood pressure, cancer and so on seem biologically plausible, says Dr Whiteman. But the nature of epidemiological research makes it hard to be sure that researchers have really thought of every other possible explanation. Biological mechanisms for many of the apparent links between sunlight and health are still missing, or poorly understood. “I don’t think we quite have proof yet,” he says. In Britain the National Institute for Health and Care Research concluded earlier this year that the evidence was not strong enough to justify altering official advice to avoid strong sunlight between March and October.
But things are changing elsewhere. Australia is full of people of European descent who are evolutionarily ill-suited to the climate they now live in, says Dr Whiteman. As a result it has some of the world’s highest rates of skin cancer, and pioneered the sun-avoidance message that has now become standard. Yet last year it tweaked its guidance to take account of the benefits of sunlight, and the importance of skin colour.
The idea that sunlight has benefits as well as risks makes for a tricky public-health message, says Antony Young, a dermatologist at King’s College London. But he nevertheless detects the beginnings of a shift in the field. “No one is saying you should get sunburnt. But some of my colleagues that have never advocated any kind of intentional sun exposure are perhaps starting to mellow.”