Douglas E. Brash, clinical professor of therapeutic radiology and dermatology at Yale School of Medical, and his colleagues discovered that skin damage from ultraviolet radiation (UV) continues hours after sun exposure. Sun or tanning beds expose the skin to UV light, which can damage the DNA in melanocytes, the melanin-making cells that give the skin its color. The research team exposed a mouse and human melanocyte cells to radiation from a UV lamp, and discovered that the melanocytes caused DNA damage immediately, but continued to do so hours after the UV exposure ended. In fact, they found that the damage occurred more after the exposure. The reason for this that the UV light activated two enzymes that combined to "excite" an electron in melanin. This process, known as chemiexcitation, generated energy that was transferred to DNA post UV exposure, creating the same DNA damage that sunlight caused in daytime.
Read more in Science Daily.