Beachgoers are familiar with the experience of spending an afternoon in the sun, going home, and noticing that their skin has changed color hours later. A new Tel Aviv University (TAU) study uncovers the science behind the mystery of why the body’s tanning process does not occur immediately after sun exposure, but only after a few hours or even days.

The researchers say that the body’s initial response is to prioritize repairing DNA damage in the skin cells, which inhibits the mechanism responsible for skin pigmentation, commonly known as tanning.

The study was led by doctoral student Nadav Elkoshi and Professor Carmit Levy of the Department of Human Molecular Genetics and Biochemistry at TAU’s Faculty of Medicine, in collaboration with a number of other researchers from TAU, Wolfson Medical Center, the Weizmann Institute of Science, the University of California, and Paris-Saclay University. It was published on May 25, 2023, in the Journal of Investigative Dermatology.

“We have two mechanisms designed to protect the skin from exposure to dangerous UV radiation,” Elkoshi explains. “The first mechanism repairs the DNA in the skin cells damaged by the radiation, while the second mechanism involves increased production of melanin, which darkens the skin in order to protect it from future exposure to radiation. In our study, we discovered why the tanning phenomenon does not occur immediately when the body is exposed to the sun, but only following a delay.

“It turns out that the mechanism that repairs our DNA takes precedence over all other systems in the cell, temporarily inhibiting the pigmentation mechanism. Only after the cells repair the genetic information to the best of their ability do they begin to produce the increased melanin.”

To test their hypothesis, the researchers activated the DNA repair mechanism in both animal models and human skin tissues. In both, a tan developed even without any exposure to UV radiation, substantiating their findings.

“The genetic information must be protected from mutations, so this repair mechanism takes precedence inside the cell during exposure to ultraviolet radiation from the sun,” Professor Levy says. “The DNA repair mechanism essentially tells all the other mechanisms in the cell, ‘Stop everything, and let me work in peace.’ One system effectively paralyzes the other, until the DNA correction reaches its peak, which occurs a few hours after the UV exposure. Only then does the pigment production mechanism get to work.

“This scientific discovery has revealed a molecular mechanism that could serve as a foundation for further research that may lead to innovative treatments that will provide maximum protection of the skin against radiation damage. In the long run, it may even contribute to the prevention of skin cancer.”