"Taking a tan in the early morning hours will lessen the risk of skin cancer," said Aziz Sancar, from the University of North Carolina, Chapel Hill, U.S. He shared the Nobel Prize in Chemistry with two others — Tomas Lindahl of Francis Crick Institute and Clare Hall Laboratory, Hertfordshire, U.K. and Paul Modrich of Howard Hughes Medical Institute and Duke University School of Medicine, Durham, U.S.
His study was on circadian rhythms and about its ability to repair DNA damage caused by UV radiation in mouse in the morning hours. But unlike mice, humans are diurnal and hence one can expect the opposite results in humans.
Dr. Sancar was awarded the Nobel Prize for mapping nucleotide excision repair — the mechanism that cells use to repair UV damage to DNA. According to a Nobel Prize release, people born with defects in this repair system will develop skin cancer if they are exposed to sunlight. The cell also utilises nucleotide excision repair to correct defects caused by mutagenic substances, among other things.
The results of a study published in the Proceedings of the National Academy of Sciences
in November 2011 found that the time of day of exposure to UV radiation is a "contributing factor" to skin cancer development in mice and "possibly in humans."
The mechanism that cells use to repair UV damage to DNA (excision repair rate) exhibits daily rhythmicity in mouse skin. In the case of mice, the ability to repair UV damage to DNA was minimum in the morning and maximum in the afternoon/evening.
As a result, mice exposed to UV rays (280-320 nm) at 4 am, when the cells ability to repair UV damage to DNA (excision repair activity) was at its lowest developed skin cancers at a "faster rate" and at about "fivefold higher frequency" compared with mice that were exposed to UV rays at 4 pm, when the excision repair activity was at its peak.
"We conclude that time of day of exposure to UV radiation is an important determinant in the carcinogenicity of UV radiation," Since the human clock is identical to the mouse but is opposite in phase (diurnal vs nocturnal), the "susceptibility of humans to UV radiation-induced skin cancers is likely to exhibit a daily rhythm as well."
"In mouse there is more DNA replication and less repair in the morning and less replication and more repair in the evening. Because UV-induced skin cancers arise from mutagenic replication of epidermal keratinocyte DNA, the same UV dose is more carcinogenic in early morning hours than when given in the early evening hours," they noted.
Dr. Sancar "predicts" that humans are less likely to develop skin cancer if exposed to UV radiation in the morning. We suspect that restricting UV radiation exposure to morning hours would reduce the risk of skin cancer in humans. It might be advisable for humans to restrict their occupational, therapeutic, recreational, and cosmetic UV radiation exposure to the morning hours.