Keep out of the sun during the afternoons when exposure to ultraviolet radiation is more damaging.

A new research has discovered that exposure to UV radiation may be less harmful to the skin than in the morning than during other times of the day as the skin's ability to repair itself rises and falls within its circadian rhythms or the body's internal clock.

According to Dr. Aziz Sancar of the University of North Carolina School of Medicine who led the research, the amount of tumors was fivefold higher in mice that were exposed to UV radiation near their bedtime. Thus, he hypothesized that in humans, exposure to UV radiation in the evening would also lead to an increase in skin cancer risk.

In an earlier work, Sancar found that XPA, a protein in humans and mice that fixes damaged DNA, follows the circadian rhythms. Levels of XPA are highest shortly after an organism awakes, which is in the evening for mice and the morning for humans. And when XPA levels are high, the protein can fix errors in DNA that are caused by UV radiation. When XPA is low, which for humans is in the evening, these errors go unrepaired, heightening cancer risk.

Sancar and his colleagues exposed mice to high levels of UV radiation at either 4 a.m. or 4 p.m. for 25 weeks to test whether the findings of XPA levels had real implications for skin cancer risk. The result showed that mice that were exposed in the morning hours, when XPA is lowest, developed five times as many tumors and these were more often invasive, meaning they were a more advanced stage of skin cancer.

"The takeaway message is that, for humans, the morning hours are safer for exposure to sunlight and tanning booths than the afternoon hours," Sancar said.

"But I don't want people to take the message from this that if you go tan in the morning, you'll be fine. The mice developed skin cancer in both groups."

The new findings were published today (Sept. 23) in the journal Proceedings of the National Academy of Sciences.