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A mouse’s levels of the DNA-repairing protein XPA are different from ours—they peak in the morning and bottom out in the evening. Researchers exposed mice to UV radiation when their XPA was at its minimum level, around 4 a.m., and others to the same rays around 4 p.m., when XPA levels peaked.[qh] 老鼠的DNA修復蛋白質XPA的水平和我們不同—它們在早晨達到峰值，夜里則從低谷回升（譯注：與后文互相矛盾，原文這里應該搞反了，應為：夜里到峰值，而早晨到最低值。參考原文后面的評論）。研究人員將兩組老鼠在不同時段，當XPA達到最低水平，凌晨4點左右，和當XPA達到最高水品，下午4點左右，置于相同的紫外線照射下。
Mice who tanned while low on the repair protein developed skin cancer faster and five times more frequently than their evening-tanning counterparts. The study is in the Proceedings of the National Academy of Sciences. [Shobhan Gaddameedhi et al, Control of skin cancer by the circadian rhythm]
Unlike mice, humans are not nocturnal, so their XPA levels rise and fall at different times. In people, XPA is at prime DNA-repairing levels in the morning, which thus looks the safest time for UV exposure. So if you want to avoid skin cancer, probably go to the tanning salon early—or better yet, don’t go at all.
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