The damage the sun inflicts on your skin may be even more insidious than was previously thought, according to new research.

A study published Thursday in Science finds that the effects of sun exposure can continue to wreak havoc on your DNA — even in the dark.

The researchers say that as much as half the harm the sun does to the DNA in skin cells occurs up to three hours after exposure via a chemical process they call the “dark pathway.”  “The main lesson of our research is that your skin doesn’t stop getting damaged when you get out of the sun,” said Douglas Brash, professor of therapeutic radiology and dermatology at Yale School of Medicine and the senior author of the paper.

Scientists have known for 50 years that exposure to UVB and UVA light negatively affects the DNA in skin cells. When a photon is absorbed by DNA, it can cause the bonds of two of the base letters on the DNA strand to fuse together. This puts an unnatural bend in the DNA and makes it difficult for it to replicate properly.

This type of damage is called a cyclobutane pyrimidine dimer (CPD) and it happens in a millionth of a millionth of a second — right after the photon hits the skin.

To make sure all their lab equipment was working correctly, the researchers exposed the cells to UV light and then measured the number of CPDs or dimers that occurred.

“The first time we did this, we thought it was a defect,” said Premi. “But we replicated it several times, using different techniques to confirm this was the real thing.”

Once the researchers determined that UV exposure really was continuing to harm DNA even after the lights went out, they decided to figure out why.

In a series of experiments they found that direct exposure to UV light causes dimers to occur in the DNA of all types of skin cells, but only melanocytes continued to accrue these defects in the absence of light.

Source: Yale School of Medicine