Sunburn is a sign of DNA damage that can lead to skin cancer

Earlier this week, new research was published about the effects of the body’s daily rhythms – known as the ‘circadian clock’ – on how skin responds to damage caused by UV exposure.

It’s an interesting piece of research, carried out in mice, but it by no means justifies the headlines it generated, some of which suggest that the findings are applicable to humans. This isn’t just incorrect, but could actually be harmful.

This isn’t to say that there’s no link between our body clocks and cancer. Our own researchers are investigating this, and are making big strides in this field.

But the way the new research, from a team in the US, has been communicated is concerning. Let’s have a look at what they actually found.

Body clocks

Mice are nocturnal – i.e. they are generally awake at night and sleep during the day. Previous research has shown that their DNA repair systems – the molecular mechanisms that protect cells from cancer-causing damage – become more active while they’re asleep (during the day), peak in the afternoon, and are less active while they’re awake, at night.

To find out more about this phenomenon, the researchers, based in North Carolina in the US, looked at what happened when they exposed mice to UV rays either at 4.00am, the end of the animals’ night, or at 4.00pm, shortly before their lights were switched off to signal the end of the day.

Mice who had their UV exposures at 4am, when their DNA repair systems were at their least active, developed cancers more quickly, and had more tumours that spread, than those who were exposed at 4pm (with fully-active DNA repair systems).

The researchers also noted that although mice are awake during the night, humans are awake during the day, so their body clocks are in opposite phases to those of mice.

From this, they concluded that humans may be more susceptible to cancers from UV exposure when it takes place in the ‘end-of-day’ slot than the ‘end-of-night’:

“We suspect that by restricting UVR exposure to morning hours would reduce the risk of skin cancer in humans.”

Stretching a point

But here’s the important point. They didn’t do any research in humans to find out whether this is actually the case. So when they concluded that they “suspect” that only being exposed to UV in the mornings may reduce the harmful effects of exposure, they made sure to say that this was a “provisional” recommendation:

“This recommendation, however, must be considered provisional until actual DNA repair rates are measured in the skin of human volunteers.”

In our view, even with the “provisional” tag, this is a bit of a stretch. There’s no research to show that similar things are going on in mouse and human skin, or that only being exposed in the morning would have any effect on cancer development..

It’s also worth noting that all but one of the UV-exposed mice, whether in the morning or evening UV groups, developed cancers. The only differences were the time it took for the tumours to develop, and the average number per mouse.


But it got worse – in the press release sent to the media, the subheading was “Study suggests that restricting sunbathing or visits to the tanning booth to morning hours would reduce the risk of skin cancer.” This is an ambiguous statement that could well be (and was) interpreted to mean that UV exposure in the morning would reduce cancer risk compared to no sun exposure, rather than compared to the same exposure in the afternoon.

Although it may sound a trivial difference, it’s led to enormous confusion in the media, with many reports sounding like they’re recommending people actively seek sun exposure in the morning to reduce the risk of skin cancer.

That’s the opposite of the real recommendation for reducing skin cancer risk.

We know from decades of quality research that getting too much exposure to UV rays increases the risk of developing skin cancer. And getting sunburnt, at any time of day, is a clear sign that the DNA in your skin’s been damaged, which can build up over time and lead to skin cancer developing in the future.

It may turn out, after more research, that the ability of the body to repair this damage changes over the course of the day, but at the moment there’s no evidence to suggest this is a real effect in humans.

And reporting research in this way threatens not just to confuse people and dilute evidence-based messages about skin cancer prevention, but in this case, could lead to people putting themselves at even higher risk by seeking more UV exposure in the morning.



  • Gaddameedhi, S., Selby, C., Kaufmann, W., Smart, R., & Sancar, A. (2011). Control of skin cancer by the circadian rhythm Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1115249108