Sunbathing may be safer in the morning due to the timing of DNA repair, a study has shown.
The skin's ability to withstand damage from ultraviolet (UV) radiation varies at different times of day, according to the research, which means skin cancer may be less likely to result from sun exposure in the morning than in the afternoon.
"Our research would suggest that restricting sunbathing or visits to the tanning booth to morning hours would reduce the risk of skin cancer in humans," said study leader Professor Aziz Sancar, from the University of North Carolina in the US.
But he added further research was needed before any "definitive recommendations" could be made and previously the same team had shown that a DNA repair protein called XPA waxed and waned throughout the day.
Damage to DNA by UV rays is the main cause of skin cancer. Each year more than 11,700 new cases of malignant melanoma, the most serious form of the disease, are diagnosed in the UK, leading to around 2,000 deaths.
The new study, published in the journal Proceedings of the National Academy of Sciences, looked at the effects of UV on mice, which also possess the XPA protein.
Mice and humans both have internal 24-hour, or circadian, clocks that influence the way their bodies work and one time-influenced biological effect shared by both species is DNA repair by XPA but since their clocks run in opposite directions, XPA in nocturnal mice is least active in the morning.
The scientists found that for mice, the risk of UV causing skin cancer was five times greater in the morning than in the afternoon and they expected the trend to be reversed in humans.
The mice were irradiated either at 4am and at 4pm. Animals exposed to UV when DNA repair was at its minimum developed tumours much faster, and five times more often, than when the repair mechanism was most active.
Professor Brian Diffey, from the Institute of Cellular Medicine at the University of Newcastle, said: "This is a fascinating study in mice. However, any extrapolation to humans is highly speculative and the nature of our exposure to sunlight and other UV sources like sunbeds is subject to many social constraints such as the split between our work and leisure time. This would make it difficult to control our exposure just to specific times of the day, assuming, of course, the findings were at all relevant to humans.