Night Shift Work and Hormone Levels in Women

Abstract

Background: Night shift work may disrupt the normal nocturnal rise in melatonin, resulting in increased breast cancer risk, possibly through increased reproductive hormone levels. We investigated whether night shift work is associated with decreased levels of urinary 6-sulfatoxymelatonin, the primary metabolite of melatonin, and increased urinary reproductive hormone levels.Methods: Participants were 172 night shift and 151 day shift-working nurses, aged 20-49 years, with regular menstrual cycles. Urine samples were collected throughout work and sleep periods and assayed for 6-sulfatoxymelatonin, luteinizing hormone (LH), follicle-stimulating hormone (FSH), and estrone conjugate (E1C).Results: 6-Sulfatoxymelatonin levels were 62% lower and FSH and LH were 62% and 58% higher, respectively, in night shift-working women during daytime sleep than in day shift-working women during nighttime sleep (P 0.0001). Nighttime sleep on off-nights was associated with 42% lower 6-sulfatoxymelatonin levels among the night shift workers, relative to the day shift workers (P < 0.0001); no significant differences in LH or FSH were observed. 6-Sulfatoxymelatonin levels during night work were approximately 69% lower and FSH and LH were 35% and 38% higher, compared with day shift workers during nighttime sleep. No differences in E1C levels between night and day shift workers were observed. Within night shift workers, 6-sulfatoxymelatonin levels were lower and reproductive hormone levels were higher during daytime sleep and nighttime work, relative to nighttime sleep (P < 0.05).Conclusions: These results indicate that night shift workers have substantially reduced 6-sulfatoxymelatonin levels during night work and daytime sleep and that levels remain low even when a night shift worker sleeps at night.Impact: Shift work could be an important risk factor for many other cancers in addition to breast cancer. MeSH Subject Headings: breast cancer, shift work, circadian rhythm, environmental carcinogens, estrogen, melatonin, 6-sulfatoxymelatonin, pineal. Cancer Epidemiol Biomarkers Prev; 21(4); 609-18. Ó2012 AACR.

Night shift work and other determinants of estradiol, testosterone, and dehydroepiandrosterone sulfate among middle-aged nurses and midwives

“…One of the proposed mechanisms underlying the increased risk of breast cancer among night shift workers was a decrease in melatonin level and an increase in reproductive hormone synthesis in response to light-atnight (13). Moreover, night workers were more likely than day workers to become obese (14,15) and to have unhealthy lifestyle (16) such as smoking (17), unhealthy diet (16) or low physical activity (18), which may contribute to the risk of cancer and affect hormonal milieu and sex hormone metabolism (19,20).…”

“…There has been a limited number of observational studies examining the potential alterations in sex hormones or their precursor DHEA among women working night shifts (13,(21)(22)(23)(24)(25)(26)(27). In a study investigating the association between the duration of night shift work and estradiol level among 663 postmenopausal women, a significant positive relationship was found (21).…”

Impact of Environmental Contaminants on Breast Cancer / Wpływ Zanieczyszczenia Środowiska Na Raka Piersi

“…Frequent night shift work was found to increase the risk for breast cancer, and higher risk was connected with longer duration of intense night shifts. Moreover, women with morning preference who worked on night shifts tended to have a higher risk than those with evening preference [297]. Long-term (≥ 30 years) night shift work in a diverse mix of occupations was associated with increased breast cancer risk (OR = 2.21, 95% CI 1.14-4.31) and this association was showing similar results for both health and non-health care workers [304].…”

“…Women who worked at night and who experienced sleep deprivation, circadian disruption and exposure to light at night were at an increased risk of breast cancer [296]. Night shift work may disrupt the normal nocturnal rise in melatonin, resulting in increased breast cancer risk, possibly through increased reproductive hormone levels [297]. A significant increase in the breast cancer risk among postmenopausal women exposed to shift work was reported by several researchers [298][299][300][301], and an increased risk of breast cancer due to longer occupational exposure to light at night at flight attendants was estimated as well [302,303].…”

24 Hours chronomics of ambulatory blood pressure and its relation with circadian rhythm of 6-sulfatoxy melatonin in night shift health care workers

“…37 Night shift work may disrupt the normal nocturnal rise in melatonin, resulting in increased breast cancer risk, possibly through increased reproductive hormone levels. 38 Women working eight or more night shifts per month had significantly lower MT6s levels than those having fewer night shifts per month (37.9 vs. 47.4 ng/mg Cr, respectively). Light exposure at night has been hypothesized as one of potential mechanisms of breast carcinogenesis in the night shift workers through inhibition of melatonin synthesis.…”