However, they note that other prospective studies are needed to further explore the relationship.
Breast cancer is the most common type of cancer among women in the US(other than skin cancer). The number of new cases of breast cancer in women was estimated to be about 212,600 in 2003, according to the National Cancer Institute.
For the melatonin study, Ruth Travis, of the British cancer charity Cancer Research UK, and colleagues compared levels of a metabolite of melatonin in the urine of 127 patients diagnosed with breast cancer with those of 353 control women to determine whether low levels of melatonin were associated with an increased risk for developing breast cancer.
They report in today's issue of the Journal of the National Cancer Institute (vol. 96, no 6, 475-482) that there was no evidence that the level of melatonin is strongly associated with the risk for breast cancer.
The hormone has been more widely investigated for its sleep-inducing effects as it is secreted by the pineal gland in the brain at night, and is an important regulator of circadian rhythms in the body, especially sleep. Supplements of melatonin are popular in the US as a treatment for jet lag and insomnia.