If you spend a day at the beach or the park and only notice a sunburn later that evening or even the next morning, it is not your imagination. According to new research by a team at Yale, exposure to UV light can continue to do damage for hours after exposure.

In a new study published in the February 19 edition of the journal Science, researchers found that light from the un or tanning beds can damage DNA in the melanocytes, which give skin its color.

At one time it was believed that melanin protected the skin by blocking UV light, however other evidence suggested that it was associated with skin damage.

Douglas E. Brash, clinical professor of therapeutic radiology and dermatology at Yale School of Medical, and his co-authors experimented with exposing mouse and human melanocyte cells to UV radiation from a lamp.

They found that the radiation caused cyclobutane dimer (CPD), a form of damage in which two DNA “letters” attach and bend the DNA. This prevents the information in the DNA from being read correctly.

Most significantly, and to the surprise of the researchers, they found that hours after UV exposure ended, the melanocytes continued to generate CPDs. Cells without melanin only generated CPDs during UV exposure and not after the fact.

According to the researchers, this means that the melanin had both protective and carcinogenic effects.

“If you look inside adult skin, melanin does protect against CPDs. It does act as a shield. But it is doing both good and bad things,” said Brash, also a member of Yale Cancer Center, in a statement.

Next the team investigated the amount of damage done after exposure by preventing normal DNA repair in mouse melanin samples. They found that half of the damage was created after, rather than, exposure. The researchers refer to this damage as “dark CPDs.”

Sanjay Premi, associate research scientist in the Brash laboratory, found that the UV light activated a pair of enzymes. The enzymes combined to excite an electron in the melanin. The energy generated, known as chemiexcitation, continued to be transferred to DNA in the dark which caused further damage.

The discovery however is not all bad news, according to the researchers. The slowness of the process may allow time for damage control such as “evening after” sunscreen. In other words, people may find some benefit in continuing to take measures to prevent sunburn for hours after UV exposure ends.

According to the Centers for Disease Control (CDC), in 2011 65,647 new cases of skin cancer were diagnosed. In that year 9,128 people died from the disease.

The CDC and the Mayo Clinic’s tips on preventing skin cancer primarily list common sense approaches such as wearing sun screen, avoiding tanning beads and direct sunlight in the middle of the day.

The American Academy of Dermatology also has information on treating sunburn, but the advice is primarily aimed at making it less uncomfortable. Because the information in this recent study is fairly new, there is no official information on preventing chemiexcitation after a sunburn.