Cancer epidemiology, biomarkers & prevention : a publication of the American Association for Cancer Research, cosponsored by the American Society of Preventive OncologyJun 19, 2008

Nighttime light exposure, melatonin levels, and sex hormone levels in postmenopausal Japanese women

Circadian Biology Weekly Brief ↗PubMed ↗DOI ↗

Updated Apr 2, 2026

Abstract

Serum estradiol levels were significantly higher in women who were not asleep at or after 1:00 a.m.

Women who worked graveyard shifts exhibited significantly increased levels of estrone.
Serum testosterone and DHEA sulfate displayed no relationship with sleeping habits or graveyard-shift work history.
Lower levels of urinary 6-sulfatoxymelatonin were observed in women awake at or after 1:00 a.m. on weekends, with borderline significance.
No significant association was found between urinary 6-sulfatoxymelatonin and any serum hormone levels.
These findings suggest that light exposure at night may be linked to breast cancer risk in postmenopausal women.

Simplified

It has been hypothesized that exposure to light at night increases the risk of breast cancer by suppressing the normal nocturnal increase 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, and exposure to light at night based on information regarding the sleeping habits and history of graveyard-shift work of 206 postmenopausal Japanese women. Serum estradiol level was significantly higher in women who were not asleep at or after 1:00 a.m. (the approximate time of the melatonin peak) than those who were asleep after controlling for covariates. Significantly increased estrone levels were observed in women who had worked graveyard shift. Serum testosterone and DHEA sulfate were unrelated to sleeping habits and history of graveyard-shift work. Urinary 6-sulfatoxymelatonin was lower in women who were not asleep at or after 1:00 a.m. on weekends than those who were asleep at this time, but the difference was of borderline significance (P = 0.08). There was no significant association between urinary 6-sulfatoxymelatonin and any serum hormone levels. These data suggest that exposure to light at night has implications for the risk of breast cancer in postmenopausal women. However, the potential role of melatonin as an intervening factor between light exposure at night and the serum concentrations of estrogen was equivocal.

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Nighttime light exposure, melatonin levels, and sex hormone levels in postmenopausal Japanese women

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