“If you really must go to a tanning booth, do it in the morning,” Aziz Sancar, from the University of North Carolina, Chapel Hill, U.S., who 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. — told New Scientist .
Dr. Sancar’s advice was based on studying how a mouse’s circadian rhythms affect its ability to repair DNA damage caused by UV radiation in the morning.
Unlike mice, humans are diurnal and hence one can expect the opposite results in humans — greater ability to repair DNA damage caused by UV radiation in the morning.
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.
His observation was based on a study undertaken in mice. 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.”
He and his colleagues arrived at this conclusion based on fact that 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,” notes the paper. 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 skin 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 note.
In the paper, Dr. Sancar “predicts” that humans are less likely to develop skin cancer if exposed to UV radiation in the morning. The authors have gone so far as stating that “it might be advisable for humans, to the extent possible, to restrict their occupational, therapeutic, recreational, and cosmetic UV radiation exposure to the morning hours.
They add a note of caution by stating that the phase of circadian rhythm is not the uniform in all people but varies between individuals. And hence any recommendation for best times for UV radiation exposure should be based on individual’s circadian rhythm. However, with rare exceptions, humans on the whole are diurnal and hence are most likely to have maximum repair capacity in the morning hours. “We suspect that by restricting UV radiation exposure to morning hours would reduce the risk of skin cancer in humans,” they write.
It is well known that the circadian clock is known to affect the onset and severity of a few other common diseases such as asthma and cardiovascular incidents.
