Cancer epidemiology, biomarkers & prevention : a publication of the American Association for Cancer Research, cosponsored by the American Society of Preventive OncologyJun 9, 2004

Levels of nighttime melatonin in women and their links to other hormones and night shift work

Circadian Biology Weekly Brief ↗PubMed ↗

Updated Mar 29, 2026

Abstract

The intraclass correlation for creatinine-adjusted 6-sulfatoxymelatonin was 0.72.

Longer durations of night work are associated with significantly increased estradiol levels, with averages of 8.8 pg/mL for non-night shift workers and 10.1 pg/mL for those working 15 or more years of night shifts.
There is a significant inverse relationship between the number of nights worked in the two weeks prior to urine collection and urinary melatonin levels (r = -0.30, P = 0.008).
No significant association was found between recent night work and estradiol levels (r = 0.10, P = 0.41).
A single morning urinary melatonin measurement may reflect long-term melatonin levels in premenopausal women.
Changes in hormone levels among women working night shifts could be linked to the increased cancer risk observed in this population.

Simplified

OBJECTIVE: Light exposure during night work suppresses melatonin production, and night work has been associated with an increased cancer risk. There is little information, however, about the interrelationships of night work, urinary melatonin levels, and levels of plasma steroid hormones in women.

METHOD: We examined the reproducibility of morning urinary measurements of 6-sulfatoxymelatonin over a 3-year period in 80 premenopausal women. We assessed correlations between average urinary melatonin and plasma steroid hormone levels and evaluated potential associations between night work and hormone levels, using current and long-term shift work information from two large, prospective cohorts, the Nurses' Health Study cohorts.

RESULTS: The intraclass correlation for creatinine-adjusted 6-sulfatoxymelatonin was 0.72 (95% confidence interval, 0.65, 0.82). We found significantly increased levels of estradiol after longer durations of night work (geometric mean levels of estradiol, 8.8 pg/mL for women who never worked night shifts versus 10.1 pg/mL for women who worked 15 or more years of night shifts; P for trend = 0.03). We observed a significant inverse association between increasing number of nights worked within the 2 weeks preceding urine collection and urinary melatonin levels (r = -0.30, P = 0.008), but no association of recent night work with estradiol (r = 0.10, P = 0.41).

CONCLUSION: A single morning urinary melatonin measurement is a reasonable marker for long-term melatonin levels among premenopausal women. Women who work on rotating night shifts seem to experience changes in hormone levels that may be associated with the increased cancer risk observed among night-shift workers.

Levels of nighttime melatonin in women and their links to other hormones and night shift work

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