New research shows the Sun can still take a toll on skin even after it sets for the night. 

Much of the damage that ultraviolet radiation (UV) imposes on our skin occurs hours after sunlight, Yale University reported. Exposure to UV light can harm the DNA in melanocytes, which are the cells that give skin its color. This type of damage is a major contributor to the development of skin cancer, which is the most common form of cancer in the U.S.

In the past, researchers believed melanin protected the skin by blocking these UV rays, but more recent studies have linked melanin with skin cell damage. To clear up these questions, a team of researchers exposed both mouse and human melanocyte cells to rays from a UV light. They found this exposure led to a type of DNA damage called cyclobutane dimer (CPD), in which two DNA "letters" bend the DNA itself, preventing its information from being read correctly. The team was shocked to see that melanocytes continued to CPDs even hours after the exposure ended, while cells that did not contain melanin only generated CPDs during UV exposure.

"If you look inside adult skin, melanin does protect against CPDs. It does act as a shield," said Douglas E. Brash, clinical professor of therapeutic radiology and dermatology at Yale School of Medicine.

The team then prevented normal DNA repair in the mouse samples in order to measure the amount of damage endured as a result of UV exposure; they observed that half of the CPDs in melanocytes were "created in the dark." Upon further investigation, the researchers found UV light activated two enzymes that "excited" an electron in melanin. The energy generated from this process, dubbed chemiexcitation, damaged DNA even after UV exposure had ended.

While the findings are concerning, the researchers believe the slowness of the chemiexcitation process could allow for the development of "evening-after" sunscreen that blocks the harmful transfer of this energy.

The findings were published Feb. 19 in the journal Nature.