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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修复蛋白质XPA的水平和我们不同—它们在早晨达到峰值，夜里则从低谷回升（译注：与后文互相矛盾，原文这里应该搞反了，应为：夜里到峰值，而早晨到最低值。参考原文后面的评论）。研究人员将两组老鼠在不同时段，当XPA达到最低水平，凌晨4点左右，和当XPA达到最高水品，下午4点左右，置于相同的紫外线照射下。
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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