Scientific Symposium - The Health Effects of Shift Work - Toronto, April 12, 2010

Light at night and health: the Nurses' Health Study cohorts

Eva S. Schernhammer (presenter) and Caroline A. Thompson


Light at night, encountered by numerous shift workers, is emerging as an occupational risk factor. This presentation will provide an overview of evidence from the Nurses’ Health Study, (NHS, 1988) and the Nurses’ Health Study II (NHS2, 1989) concerning the health effects of exposure to light at night. The unique strengths of the NHS cohorts include a prospective study design, detailed confounder information, large size, and a high prevalence of female night workers. Limitations include the one-time assessment of shift work in NHS, loss to follow-up (though small in both cohorts), potential errors in exposure assessment, and the limited ability to study rare diseases.

Several studies in these cohorts using the shift work information have yielded valuable results for various endpoints. For cancer risk, significant associations were observed for breast, colon, and endometrial cancer risk. Moderate increases in cancer risk were observed in women who had worked rotating shifts for longer durations (i.e., 15+ years). All cancer studies within the two NHS cohorts controlled for major risk factors of each individual cancer as well as job stress and SES. Evidence from the NHS cohorts contributed to the IARC recommendation to classify night shift work a carcinogen class 2A (probable carcinogen).

Cardiovascular disease risk was also found to be elevated in nurses participating in the NHS. Bone health might be affected by night work, as well as menstrual cycle regularity and menopause, all of which have recently been studied in the NHS cohorts. Those aspects of light exposure in rotating shift workers most strongly related to circadian system response, as measured by melatonin levels in urine, is currently being evaluated in NHS2, using a circadian light meter that captures the short wavelength portion of visible light. Short wavelength light has been described to most strongly suppress melatonin levels.

Determining what aspects of shift schedules (such as shift length, frequency of rotation, number of hours worked per week) are most detrimental to health is the next frontier in shift work and disease prevention. In 2009, we have added a battery of shift work questions to the NHS2 questionnaire to assess specific aspects of the nurses’ work schedule and changes throughout their professional career. Other factors of potential relevance to circadian disruption were assessed, including morningness-eveningness and sleep duration during each of the various lifetime periods. We anticipate that future research will benefit from a clear and complete description of how work schedules differentially relate to health.

Presenter Biography

Eva Schernhammer, MD, DrPH, is an Assistant Professor in the Department of Epidemiology, Harvard Medical School. Dr. Schernhammer obtained her MD from the Medical University of Vienna, an MSc in Psychology from the University of Vienna, and a Masters degree and DrPH from the Harvard University School of Public Health.

Her primary research interest is in exploring the exposures that influence the circadian system in humans and the health consequences of these exposures. Current projects study the role of clock genes in the association between shift work and cancer and melatonin's cancer preventive potential. Another research focus is to examine the role of other endogenous hormones such as IGF-I and IGFBP-3, and differences with respect to functionally different polymorphisms of these genes, and risk of breast cancer.

Dr. Schernhammer is also interested in the etiology and prevention of gastrointestinal tumors and in studying the relation of Parkinson's disease with cancer etiology, to further understand biological mechanisms in the development of cancer in humans.



Schernhammer E, Thompson CA



 Occupational Cancer Research CentreInstitute for Work & Health | Research Excellence Advancing Employee Health