Cohort study of cancer risk among male and female shift workers

Abstract

Objectives Melatonin, a hormone that inhibits experimentally induced cancers, is suppressed by nighttime exposure to light so that nighttime shift workers may be at an increased risk of cancer. Previous studies of shift workers found an increased risk of breast cancer among women and suggested a possible increased risk of colon cancer among women and prostate cancer. The present study was conducted to see whether these previous findings could be confirmed and whether shift workers are at elevated risk for cancer at additional sites.Methods Altogether 2 102 126 male and 1 148 661 female workers were identified who worked in both 1960 and 1970. Their jobs were classified according to the percentage of shift workers, and they were followed from 1971 through 1989 or until they were diagnosed with cancer or died. Standardized incidence ratios (SIR) were used to compare the adjusted cancer incidence rates for shift workers with those for nonshift workers.Results Cancer rates were not elevated for the male shift workers [all sites combined: N=6524 cases among shift workers, SIR 1.02, 95% confidence interval (95% CI) 1.00-1.05; prostate: N=1319, SIR 1.04, 95% CI 0.99-1.10] or for the female shift workers (all sites combined: N=268, SIR 1.00, 95% CI 0.89-1.13; breast: N=70 cases, SIR 0.94, 95% CI 0.74-1.18).Conclusions No evidence was found for an association between shift work and breast or prostate cancer, or all cancer sites combined among shift workers.

Sleep duration and the risk of prostate cancer: the Ohsaki Cohort Study

“…In relation to melatonin, there have been several observational studies of night work, shift work or visual impairment, and sex hormone-related cancers such as prostate or breast (Feychting et al, 1998;Verkasalo et al, 1999;Kliukiene et al, 2001;Megdal et al, 2005;Kubo et al, 2006;Conlon et al, 2007;Schwartzbaum et al, 2007). However, there has been no study of sleep duration and prostate cancer risk, even though prostate cancer, like breast cancer, is also a sex hormone-related cancer.…”

“…Pinheiro et al, 2006). Melatonin, which is secreted mainly from the pineal gland and plays a role in sleep duration, is suggested as one of the candidates responsible for the association in breast cancer (Brzezinski, 1997;Schernhammer and Schulmeister, 2004) as it influences the synthesis and secretion of sex hormones by promoting the release of gonadotropin-releasing hormone (Martin and Klein, 1976;Aleandri et al, 1996).In relation to melatonin, there have been several observational studies of night work, shift work or visual impairment, and sex hormone-related cancers such as prostate or breast (Feychting et al, 1998;Verkasalo et al, 1999;Kliukiene et al, 2001;Megdal et al, 2005;Kubo et al, 2006;Conlon et al, 2007;Schwartzbaum et al, 2007). However, there has been no study of sleep duration and prostate cancer risk, even though prostate cancer, like breast cancer, is also a sex hormone-related cancer.…”

Sleep duration and the risk of breast cancer: the Ohsaki Cohort Study

“…This is because melatonin suppresses the synthesis and secretion of sex hormones by promoting the release of gonadotropin-releasing hormone (Martin and Klein, 1976;Aleandri et al, 1996). In relation to melatonin secretion, there have been several observational studies on night work or visual impairment and breast cancer (Feychting et al, 1998;Verkasalo et al, 1999;Kliukiene et al, 2001;Schernhammer et al, 2001;Megdal et al, 2005;Pukkala et al, 2006;Schwartzbaum et al, 2007). In addition, there have been three prospective cohort studies of sleep duration and the risk of breast cancer, although with inconsistent findings (Verkasalo et al, 2005;Pinheiro et al, 2006;Wu et al, 2008).…”

Malignant Tumors of the Female Reproductive System

“…A cohort study of professional firefighters in Florida reported a 5-fold increased risk of cervical cancer, unadjusted for lifestyle habits [55]. A Swedish registry-based cohort study found a 39% non-significant increase in risk associated with shift work (SIR 1.39, 95% CI 0.82-2.19), however, the definition of shift work used in the study was very rough, including occupations in which at least 40% of the workers reported working rotating shifts (3 shifts per day), or workers who worked at least 1 night in the week preceding the interview [56]. A Finnish record-linkage study reported excess risks of cervical cancer of about 20 to 40% with exposures to aliphatic and alicyclic, aromatic, and chlorinated hydrocarbon solvents.…”

“…A Swedish registrybased cohort study did not find increased risks associated with shift work, but the definition of shift work used in the study was very rough, including occupations in which at least 40% of the workers reported working rotating shifts (3 shifts per day), or workers who worked at least 1 night in the week preceding the interview [56]. A cohort study in the US reported an increased risk among nurses who worked at least 20 years in rotating shifts; the risk was larger in a subgroup of obese nurses, after adjustment for potential confounding factors (body mass index > 30 kg/m 2 ; RR 2.09, 95% CI 1.24-3.52), and increased with the duration of shift work [64].…”