Exposure to light at night, melatonin, and risk of breast cancer.

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There has been a hypothesis that exposure to light at night may increase a risk of breast cancer by suppressing the normal nocturnal rise in melatonin production and release, thereby resulting in increased levels of circulating estrogen. We conducted a case-control study based on 178 cases and 433 controls to assess the association between light-at-night exposure and the risk of breast cancer. Cases were 178 women newly diagnosed as breast cancer. Controls were 433 women who were selected from those attended a mammographic breast cancer screening at the same hospital and confirmed to have no breast cancer. Women who did not sleep around 1 and 2 a.m. (the time of the approximate melatonin peak) during weekend, but not during weekday, tended to be at an increased risk of breast cancer, but the association was not statistically significant. Women who had a history of night-work were at a significantly increased risk of breast cancer. We also evaluated interrelationships among concentrations of the principal metabolite of melatonin in urine, 6-sulfatoxymelatonin (aMT6-s) and serum estrogen and androgen and exposure to light at night. Urinary aMT6-s but not any of serum hormones were significantly lower in women who did not sleep around 1 and 2 a.m. during weekday than those who slept after controlling for covariates. There were no significant associations between urinary aMT6-s levels and serum hormone levels. It is interesting to clarify the determinants of urinary melatonin level. We found that vegetable intake was significantly positively associated with urinary melatonin levels. The melatonin contained in plants may have contributed to the increase of circulating melatonin level