UV Exposure May Be Less Damaging in the Morning

“If you really must go to a tanning booth, do it in the morning,” says Nobel Laureate Aziz Sancar, from the University of North Carolina, Chapel Hill, U.S.

A tanning booth is a device usually used for the purpose of cosmetic tan. They use very high output lamps that emits UV rays which is similar to Mediterranean sun during mid day.

People who get a lot of UV exposure are at greater risk for skin cancers. Our body has their own repair mechanism protecting us from DNA damage. Nucleotide excision repair, a mechanism where cells use to repair the DNA damage induced by ultraviolet rays (UV). Humans are most likely to have maximum repair capacity in the morning hours.

But studies have shown that circadian rhythm affects the ability to repair DNA damage caused by UV radiation. This has been proved by Dr. Sancar and colleagues which has been published in the Proceedings of the National Academy of Sciences in November 2011.

Circadian rhythms are physical, mental and behavioral changes that follow a roughly 24-hour cycle, responding primarily to light and darkness in an organism’s environment.

From their study, it was found that XPA, a protein in humans and mice that fixes damaged DNA, follows the circadian rhythm.

They did this study in mice by exposing them to UV rays (280-320 nm) at 4 am, developed skin cancers at a “faster rate” by a 5 fold increase compared with mice that were exposed to UV rays at 4 pm, when the excision repair activity was at its peak.

Levels of XPA are highest shortly after an organism awakes, which is in the evening for mice and morning for humans. And when XPA levels are high, the protein can fix errors in DNA that are caused by UV radiation.

This observation showed that “the time of day of exposure to UV radiation is a contributing factor” to skin cancer development in mice and humans.

Thus, mice being nocturnal have their excision repair activity at its peak in the evening while their mutagenic replication of epidermal keratinocyte DNA is less. On the other hand, they have minimal repair activity in the morning.

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.” However, with rare exceptions, humans being diurnal are most likely to have maximum repair capacity in 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.

“We suspect that by restricting UV radiation exposure to morning hours would reduce the risk of skin cancer in humans,” they write.

More on this can be read here.

Categories: Cancer, Healthcare, Research

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