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Morning tans may cause less skin damage

UNC CHAPEL-HILL (US) — Sunbathing or visiting a tanning booth during the morning hours, rather than afternoon, may lower the risk of skin cancer, new research suggests.

The findings by researchers at the University of North Carolina at Chapel Hill indicate the timing of exposure to UV rays may influence the onset of skin cancer. The study, performed in mice, found that exposure to UV radiation in the morning increased the risk of skin cancer by 500 percent over identical doses in the afternoon.

Although mice and humans both reside on a 24-hour day, the “circadian” clocks of these nocturnal and diurnal creatures run counter each other. This key difference in biology means that humans are most protected from the sun’s harmful rays when mice are most susceptible, and vice versa.

“Therefore, our research would suggest that restricting sunbathing or visits to the tanning booth to morning hours would reduce the risk of skin cancer in humans,” says senior study author Aziz Sancar, professor of biochemistry and biophysics. “However, further studies in humans are needed before we can make any definitive recommendations.”

Sancar has previously shown that a protein called XPA, responsible for repairing the DNA damage wrought by UV radiation, waxes and wanes throughout the day. In a study published this week in the Proceedings of the National Academy of Sciences, he and colleagues looked to see if the cyclical nature of this DNA repair molecule had an influence on the onset of skin cancer.

They exposed two groups of mice to UV radiation—either at 4 a.m. or at 4 p.m.—and waited for cancer to develop. Mice irradiated when the repair activity was at its minimum developed tumors much faster and at five-fold higher frequency compared with mice exposed to UV when the protein’s repair function was at its maximum.

The researchers predict that humans will have a higher rate of DNA repair in the morning and would be less prone to the carcinogenic effect of UV radiation in the morning hours. They plan to measure actual DNA repair rates in the skin of human volunteers to confirm that morning sun is safest for humans.

The research was funded by the National Institutes of General Medical Sciences and the National Institute of Environmental Health Sciences. Researchers from North Carolina State University collaborated on the work.

More news from UNC-Chapel Hill: http://www.med.unc.edu/

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