Rotating night shift schedules may speed early menopause

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A recent study of the association between rotating night shift schedules and age at menopause suggests a modest acceleration of reproductive senescence in a large national cohort of nurses. The risk may be greater in women under age 45 and in those who work rotating night shifts frequently.

According to the study, published in a recent issue of Human Reproduction, this is the first prospective investigation of the association of night work with menopausal timing. “Due to the complexity of central reproductive regulation, competing mechanisms support that women working more rotating night shifts over their reproductive lifetime could experience either delayed or advanced menopausal onset,” the authors said.

The cohort study included 80,840 women in Nurses’ Health Study 2 (NHS2) and follow-up was from 1991 to 2013. The participants were registered nurses in the United States aged 25 to 42 when the study began in 1989. Every other year for 22 years they sent in questionnaires that included information on lifestyle, medical conditions and use of medications, as well as other environmental and occupational exposures. 

Women in menopause were excluded as were those who used menopausal hormone therapy or had been diagnosed with cancer (other than non-melanoma skin cancer) on, or prior to returning the 1993 questionnaire. Women were also excluded if they did not return the baseline 1991 questionnaire or had missing or incomplete records for their rotating night shift work exposure at, or before the study baseline.

According to the report, 34% of women studied had reached natural menopause at follow-up. Participants who worked 20 months or more of rotating night shifts in the previous 2 years had an increased risk of earlier menopause (multivariable-adjusted (MV)-HR = 1.09, 95% CI: 1.02–1.16) than those who did not do rotating night shift work. 

The risk was higher in women undergoing menopause or otherwise censored under age 45 (MV-HR = 1.25, 95% CI: 1.08–1.46), than it was for those continuing in the study who were older than age 45 (MV-HR = 1.05, 95% CI: 0.99–1.13). Higher risk of menopause in women who reached menopause before age 45 was associated with working 10 or more years of cumulative rotating night work (MV-HR10–19 years = 1.22, 95% CI: 1.03–1.44; MV-HR  ≥20 years = 1.73, 95% CI: 0.90–3.35), but the association between menopause and night work was not seen in women older than age 45 (MV-HR10–19 years = 1.04, 95% CI: 0.99–1.10; MV-HR ≥ 20 years = 1.01, 95% CI: 0.89–1.15).

The researchers used cox proportional hazards models to estimate hazard ratios for menopause, adjusted for age, smoking status, body mass index, physical activity, alcohol consumption, reproductive factors, and exogenous hormone use.

“So far, we can only speculate the degree to which disruptive circadian stimuli, through cascade effects on central and peripheral HPOA targets, including nocturnal melatonin suppression, may play a role,” the authors said. “Further study of the interplay between circadian signaling and biological mediators of stress among shift workers, in addition to validation of this finding among studies using more detailed shift schedule data, is recommended.” 

Results of the study suggest that women who already may be prone to earlier menopause may further shorten their reproductive years by working rotating night shifts. In addition to reducing a woman’s reproductive life, earlier menopause increases risk of cognitive decline, osteoporosis, cardiovascular disease, and a shorter lifespan.