Nightshift work and fracture risk: the Nurses' Health Study

Nightshift work suppresses melatonin production and has been associated with an increased risk of major diseases including hormonally related tumors. Experimental evidence suggests that light at night acts through endocrine disruption likely mediated by melatonin. To date, no observational study has addressed the effect of night work on osteoporotic fractures, another condition highly sensitive to sex steroid exposure. Our study, to our knowledge, the first to address this question, supports the hypothesis that nightshift work may negatively affect bone health, adding to the growing list of ailments that have been associated with shift work. We evaluated the association between nightshift work and fractures at the hip and wrist in postmenopausal nurses. The study population was drawn from Nurses' Health Study participants who were working full or part time in nursing in 1988 and had reported their total number of years of rotating nightshift work. Through 2000, 1,223 incident wrist and hip fractures involving low or moderate trauma were identified among 38,062 postmenopausal women. We calculated multivariate relative risks (RR) of fracture over varying lengths of follow-up in relation to years of nightshift work. Compared with women who never worked night shifts, 20+ years of nightshift work was associated with a significantly increased risk of wrist and hip fractures over 8 years of follow-up [RR = 1.37, 95% confidence interval (CI), 1.04-1.80]. This risk was strongest among women with a lower body mass index (< 24) who never used hormone replacement therapy (RR = 2.36; 95% CI, 1.33-4.20). The elevated risk was no longer apparent with 12 years of follow-up after the baseline single assessment of nightshift work. Long durations of rotating nightshift work may contribute to risk of hip and wrist fractures, although the potential for unexplained confounding cannot be ruled out.