Urinary 6-sulfatoxymelatonin, light exposure at night, and serum estrogens and androgens in postmenopausal Japanese women.

B204

There has been a hypothesis that exposure to light at night may increase the risk of breast cancer by suppressing the normal nocturnal rise in melatonin production and release, thereby resulting in increased levels of circulating estrogen. We assessed associations among concentrations of serum estrogen and androgen and the principal metabolite of melatonin in urine, 6-sulfatoxymelatonin (aMT6-s), and exposure to light at night based on information about the sleeping habits of 161 postmenopausal Japanese women. Urinary aMT6-s was significantly lower in women who were not asleep around 1:00 and 2:00 a.m. (the approximate time of the melatonin peak) on weekdays than those who were asleep after controlling for age, body mass index, alcohol intake, and the day length of the day previous to the urine collection. Significantly increased estradiol levels were observed in women who were not asleep around 1:00 and 2:00 a.m. on weekdays and weekends, after controlling for the covariates. There were no significant associations between urinary aMT6-s levels and serum hormone levels. We could not find any evidence of a link between melatonin, exposure to light at night, and serum estrogen and androgen. However, the results should be regarded as preliminary because of the limitations of a small sample size.