CORRESPONDENCE


 

RESPONSE: Re: Night Shift Work, Light at Night, and Risk of Breast Cancer

Eva Schernhammer, Graham Colditz

Affiliations of authors: E. Schernhammer, Channing Laboratory, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, MA; G. Colditz, Department of Epidemiology, Harvard School of Public Health, Channing Laboratory, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Harvard Center for Cancer Prevention, and Epidemiology Program, Dana-Farber/Harvard Cancer Center, Boston.

Correspondence to: Eva Schernhammer, M.D., M.P.H., Channing Laboratory, 181 Longwood Ave., Boston, MA 02115 (e-mail: eva.schernhammer{at}channing.harvard.edu).

We thank Dr. Spiegel et al. for their interesting contribution.

The elevated breast cancer risk observed among women who work rotating night shifts (13) stimulates novel hypotheses for underlying biologic causes of potential health risks associated with exposure to artificial light at night. One avenue is to view work at night as a proxy for other potential risk factors for breast cancer, such as stress and related cortisol levels. For example, studies have shown that cortisol levels are low during the night shift but that married nurses had higher cortisol levels than single nurses (4). Such observations support the notion that domestic stress, possibly mediated through role conflicts among married nurses, alters cortisol levels in shift workers. Dr. Spiegel's and recent other work have implicated that altered cortisol levels predict survival time among breast cancer patients. Yet, their appealing theory that a healthy woman's breast cancer risk may also be elevated through altered cortisol levels (caused by sleep deprivation and anticipated higher stress levels among shift workers) still requires confirmation.

Alternatively, there is evidence regarding the responsiveness of the pineal gland to changes in emotional state, thereby inducing alterations in melatonin levels. Rodent studies suggest that the suppression of melatonin by light could be modified by changes in emotional state, produced by adversive conditioning. On the contrary, in humans, activation induced by physical stress in the middle of the dark phase did not alter melatonin levels (5).

The strongest biologic evidence to date supports cancer risk emerging directly from the exposure to light at night through the melatonin pathway. Artificial light was repeatedly shown to profoundly suppress humans' melatonin levels and, in particular, those of women (6). On average, melatonin concentrations decrease by approximately 35% after 2 weeks of intermittent nightly light exposures. Moreover, the widely established antiproliferative effect of melatonin through its antioxidative activity and potential other immunomodulating mechanisms is not only limited to modulations on estrogen receptors, thereby affecting breast cancer risk, but has already been extended to other cancers.

Based on these novel findings, we propose that the exposure to light at night and subsequent melatonin suppression may also affect other cancers in humans. We believe that our hypothesis is given further importance by recent evidence for a unique photopigment in the human eye that mediates circadian photoreception. The new photoreceptor influences the biologic effects of light, specifically, the control of the hormone melatonin (7), a phenomenon that has been poorly understood until very recently. It was further determined that wavelengths of light in the blue region of the visible spectrum (e.g., fluorescent light, commonly used in environments with artificial light) have the highest potency in causing changes in melatonin levels.

Light at night is one of the most common occupational exposures in our industrialized societies. Therefore, further research to improve our current understanding of the effects of light at night on human health is prudent.

REFERENCES

1 Hansen J. Increased breast cancer risk among women who work predominantly at night. Epidemiology 2001;12:74–7.[Medline]

2 Davis S, Mirick DK, Stevens RG. Night shift work, light at night, and risk of breast cancer. J Natl Cancer Inst 2001;93:1557–62.[Abstract/Free Full Text]

3 Schernhammer ES, Laden F, Speizer FE, Willett WC, Hunter DJ, Kawachi I, et al. Rotating night shifts and risk of breast cancer in women participating in the nurses' health study. J Natl Cancer Inst 2001;93:1563–8.[Abstract/Free Full Text]

4 Goldstein IB, Shapiro D, Chicz-DeMet A, Guthrie D. Ambulatory blood pressure, heart rate, and neuroendocrine responses in women nurses during work and off work days. Psychosom Med 1999;61:387–96.[Abstract/Free Full Text]

5 Monteleone P, Maj M, Fuschino A, Kemali D. Physical stress in the middle of the dark phase does not affect light-depressed plasma melatonin levels in humans. Neuroendocrinology 1992;55:367–71.[Medline]

6 Graham C, Cook MR, Gerkovich MM, Sastre A. Examination of the melatonin hypothesis in women exposed at night to EMF or bright light. Environ Health Perspect 2001;109:501–7.

7 Brainard GC, Hanifin JP, Greeson JM, Byrne B, Glickman G, Gerner E, et al. Action spectrum for melatonin regulation in humans: evidence for a novel circadian photoreceptor. J Neurosci 2001;21:6405–12.[Abstract/Free Full Text]



             
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