Night-shift work and Light-at-Night

At a Glance

Shift work that disrupts circadian rhythms is considered a probable human carcinogen, based in large part on the growing association between shift working and increased incidence of breast cancer.  A major explanation for this relationship is that exposure to light-at-night, common for night shift workers, can lead to a decrease in the secretion of melatonin and can increase the risk of developing breast cancer.

What is light-at-night?

Light-at-night is exposure to light during nighttime hours when it is normally dark. This exposure is common for evening and night shift workers, as well as for people living in urban areas where there is significant light pollution. Exposure to light-at-night has been linked to an increased risk of developing breast cancer.[1],[2]

What evidence links light-at-night to breast cancer?

  • In 2007, the International Agency for Research on Cancer concluded that shift work that disrupts circadian rhythms is probably carcinogenic to humans, based in large part on the growing association between shift working and increased incidence of breast cancer.[3]
  • One study of occupation and cancer in Britain calculated that for 2012, night shift work was linked with 4.5 percent of breast cancer diagnoses and deaths.[4]
  • In a study of Danish nurses, effects of night shift work on breast cancer risk were greatest for women who worked rotating hours that included the overnight (as opposed to the evening) shift, and for those who worked 12-hour shifts that alternated day and night work, as compared to shorter work periods.[5]
  • Risk of developing breast cancer increased for women who worked night shifts for more than 4.5 to 5 years, and for those who regularly engaged in night work for at least four years prior to their first pregnancy, thus before the time when their mammary cells had fully differentiated.[6]
  • Exposure to ambient artificial light in the bedroom at night (as a result of light pollution in urban areas) can contribute to a disrupted circadian rhythm and can increase the risk of breast cancer.[7],[8]
  • Increasing exposure to bright indoor light at night decreases the secretion of melatonin by the pineal gland.[9] Melatonin secretion is important for regulating pituitary and ovarian hormones, as well as for maintaining an overall healthy metabolism.[10] The same genes that mediate the protective effects of melatonin are also thought to be involved in the development of breast cancer.[11] Several studies have found evidence that high levels of melatonin may have a protective effect against breast cancer, while the decrease in melatonin secretion as a result of exposure to light during night shift work is implicated in increasing the risk for breast cancer:
    • Increased tumor levels as well as incidence of metastatic cancer in humans were associated in clinical studies with low levels of melatonin.[12]
    • In rodent models, higher levels of melatonin have been associated with decreased incidence and size of mammary tumors, and when they do occur, the cancers appear to develop later.[13]
    • In human mammary tumors that had been grafted into mice, injections of blood from women that were taken at night (when melatonin is high) decreased proliferation and growth of mammary tumors, as compared to the use of samples collected during the day, when melatonin levels are naturally lower.[14]
  • In addition to disruptions in melatonin production, other possible consequences of shift work may be linked to increased cancer rates, and require further study.
    • Chronic disruption of the timing of sleep (phase shift sleep disruption)[15]
    • Changes in metabolism[16]
    • Desynchronization between the central and peripheral nervous system[17]
    • Decreased vitamin D production[18]

Some reports have not found significant evidence linking light exposure during night shift work to breast cancer incidence. Reasons for differing results may come from methodological differences between studies, such as varied definitions of “shift work” and “night.”

Who is most likely to be exposed to light-at-night?

Occupations requiring night shift work, particularly overnight shifts, expose employees to light during the night hours, and these occupations may increase individuals’ risk of developing breast cancer. Evidence has been found linking night shift work and increased incidence of cancer in occupations such as nursing and airline work (specifically flight attendants).[19],[20]

Who is most vulnerable to the health effects of light-at-night?

Night shift work may affect individuals’ levels of melatonin differently depending on race and ethnic background.[21] Studies have found that Asian and Asian American women who work night shifts actually have less melatonin suppression than white women.[22] The effects of night shift work may also be related to breast tumor type. One study found that night shift work increased the risk of breast cancer, and specifically increased the likelihood of hormone-receptor-positive pre-menopausal breast cancer.[23]

What are the top tips to avoid exposure?

  • If possible, limit shift work to a few years.
  • If sleeping during daylight, use blackout curtains or window shades and wear an eye mask to create a totally dark environment.
  • Use blackout curtains or window shades to reduce ambient nighttime light pollution, especially in bright urban environments.
  • Create a sleeping environment free from electronic lights.
  • Support research into the best patterns of shift work.

[1] Stevens, R.G. (2009). Working against our endogenous circadian clock: Breast cancer and electric lighting in the modern world. Mutat Res, 680:106-8.

[2] Cos, S., Sánchez-Barceló, E.J. (2000). Melatonin and mammary pathological growth. Front Neuroendocrinol, 21:133-70.

[3] International Agency for Research on Cancer (2007). Shiftwork. IARC Monogr Eval Carcinog Risks Hum.

[4] Slack, R., Young, C., Rushton, L., British Occupational Cancer Burden Study Group. (2012). Occupational cancer in Britain. Female cancers: breast, cervix and ovary. Br J Cancer 107, Suppl 1:S27-32.

[5] Hansen, J., Stevens, R.G. (2012). Case-control study of shift-work and breast cancer risk in Danish nurses: impact of shift systems. Eur J Cancer, 48:1722-9.

[6] Menegaux, F., Truong, T., Anger, A., et al. (2013). Night work and breast cancer: a population-based case-control study in France (the CECILE study). Int J Cancer, 132:924-31.

[7] Blask, D.E., Hill, S.M., Dauchy, R.T., et al. (2011).Circadian regulation of molecular, dietary, and metabolic signaling mechanisms of human breast cancer growth by the nocturnal melatonin signal and the consequences of its disruption by light at night. J Pineal Res, 51:259-69.

[8] Stevens, R.G. (2009). Working against our endogenous circadian clock: Breast cancer and electric lighting in the modern world. Mutat Res, 680:106-8.

[9] Cos, S., González, A., Güezmes, A., et al. (2006). Melatonin inhibits the growth of DMBA-induced mammary tumors by decreasing the local biosynthesis of estrogens through the modulation of aromatase activity. Int J Cancer, 118:274-8.

[10] Cos, S., Sánchez-Barceló, E.J. (2000). Melatonin and mammary pathological growth. Front Neuroendocrinol, 21:133-70.

[11] Zhu, Y., Stevens, R.G., Hoffman, A.E., et al. (2011). Epigenetic impact of long-term shiftwork: pilot evidence from circadian genes and whole-genome methylation analysis. Chronobiol Int, 28:852-61.

[12] Cos, S., Sánchez-Barceló, E.J. (2000). Melatonin and mammary pathological growth. Front Neuroendocrinol, 21:133-70.

[13] Cos, S., Sánchez-Barceló, E.J. (2000). Melatonin and mammary pathological growth. Front Neuroendocrinol, 21:133-70.

[14] Blask, D.E., Hill, S.M., Dauchy, R.T., et al. (2011).Circadian regulation of molecular, dietary, and metabolic signaling mechanisms of human breast cancer growth by the nocturnal melatonin signal and the consequences of its disruption by light at night. J Pineal Res, 51:259-69.

[15] Anjum, B. (2012). Associations of circadian disruption of sleep and nutritional factors with risk of cancer. Open Nutraceuticals Journal, 5:124-35.

[16] Aronson, K,, Grundy, A., Korsiak, J., Spinelli, J.J. (2015). Causes of breast cancer: could work at night really be a cause? Breast Cancer Management, 4:125-7.

[17] Fritschi, L., Glass, D.C., Heyworth, J.S., et al. (2011). Hypotheses for mechanisms linking shiftwork and cancer. Med Hypotheses, 77:430-6.

[18] Aronson, K,, Grundy, A., Korsiak, J., Spinelli, J.J. (2015). Causes of breast cancer: could work at night really be a cause? Breast Cancer Management, 4:125-7.

[19] Hansen, J., Stevens, R.G. (2012). Case-control study of shift-work and breast cancer risk in Danish nurses: impact of shift systems. Eur J Cancer, 48:1722-9.

[20] He, C., Anand, S.T., Ebell, M.H., Vena, J.E., Robb, S.W. (2015). Circadian disrupting exposures and breast cancer risk: a meta-analysis. Int Arch Occup Environ Health, 88:533-47.

[21] Pronk, A., Ji, B-T, Shu, X-O, et al. (2010). Night-shift work and breast cancer risk in a cohort of Chinese women. Am J Epidemiol, 171:953-9.

[22] Bhatti, P., Mirick, D.K., Davis, S. (2013). Racial differences in the association between night shift work and melatonin levels among women. Am J Epidemiol, 177:388-93.

[23] Papantoniou K, Castaño-Vinyals G, Espinosa A, Aragonés N, Pérez-Gómez B, Ardanaz E, Altzibar JM, Sanchez VM, Gómez-Acebo I, Llorca J, Muñoz D. Breast cancer risk and night shift work in a case–control study in a Spanish population. European journal of epidemiology. 2015 Jul 24:1-2.

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