Light exposure at night, sleep duration, melatonin, and breast cancer

Abstract

Evidence from observational studies on light at night (LAN) exposure, sleep duration, endogenous melatonin levels, and risk for breast cancer in women is conflicting. This led us to conduct a dose-response analysis of published observational data. Pertinent studies were identified by searching Medline, Web of Science, and EMBASE through April 2013. The dose-response relationship between sleep duration, urinary 6-sulphatoxymelatonin levels, and breast cancer was assessed using the restricted cubic spline model and by multivariate random-effects metaregression. A separate meta-analysis was also carried out to calculate the relative risks (RRs) with 95% confidence intervals (CIs) for breast cancer for the comparable categories or highest levels of exposure versus the lowest levels. Twelve case-control and four cohort studies were included in the analysis. High artificial LAN exposure is associated with an increased risk for breast cancer (RR=1.17, 95% CI: 1.11-1.23), but not ambient LAN exposure (RR=0.91, 95% CI: 0.78-1.07). The summary RR for breast cancer is 1.00 (95% CI: 0.995-1.01) for an increment of 1 h of sleep per night. No significant dose-response relationship between sleep duration and breast cancer was found either for the linearity test (Ptrend=0.725) or for the nonlinearity (Ptrend=0.091) test. An increasein of 15 ng/mg creatinine in urinary 6-sulphatoxymelatonin is associated with a 14% reduced risk for breast cancer (RR=0.86, 95% CI: 0.78-0.95), with a linear dose-response trend (Ptrend=0.003). There was no evidence of substantial heterogeneity or publication bias in the analysis. Our study adds to the evidence of LAN breast cancer theory. Further research in this area is warranted.

Breast cancer and exposure to aircraft, road, and railway-noise: a case–control study based on health insurance records

“…Evidence from such studies led the International Agency for Research on Cancer (IARC) to declare shiftwork with circadian disruption to be "probably carcinogenic to humans (Group 2A)" (5). However, pooled breast cancer risks of female night shift workers based on meta-analyses of available studies vary and range from a 5-51% increase in risk (6)(7)(8)(9)(10)(11)(12)(13)(14). While an association between breast cancer and working nights may be due to the effect of LAN on melatonin levels, other aspects of working at night, such as reduced sleep may also be involved in tumor development or growth.…”

Sleep and circadian disruption and incident breast cancer risk: An evidence-based and theoretical review

“…Two more recent meta-analyses focusing exclusively on breast cancer found no evidence that sleep duration was associated with increased incidence (Yang et al, 2014; Qin et al, 2014). Both reviewed the same six studies, all that had been published on sleep duration and breast cancer through 2013.…”

“…Eight of the meta-analyses reported an overall increased incidence of breast cancer (ranging from 5 to 48%) in women exposed to some form of circadian disruption, including lifetime exposure to night shift work and LAN (He et al, 2015a; Megdal et al, 2005; Jia et al, 2013; Kamdar et al, 2013; Wang et al, 2013; Ijaz et al, 2013; Yang et al, 2014; Lin et al, 2015). Furthermore, six of the metaanalyses conducted sub-analyses on years and nights of exposure, with converging evidence from five of the six studies suggesting that circadian disruption is associated with increased risk in a dose-dependent manner (Wang et al, 2013; Ijaz et al, 2013; Yang et al, 2014; Lin et al, 2015; He et al, 2015b). Kamdar and colleagues (Kamdar et al, 2013) did not find that participants working 8 or more years of NSW were at greater risk for developing breast cancer compared to non-shift workers, and Jia and colleagues (Jia et al, 2013) found that women working 15 or more years of NSW were not at increased risk above and beyond women who reported ever working night shift.…”

Optimising sleep for night shifts

“…Theoretically, exposure to bright light during the shift might reduce sleepiness by inhibiting the secretion of the sleep-promoting circadian hormone melatonin, but problems with trial designs have led to inconclusive results 82122. Furthermore, diminished night time levels of melatonin, which also functions as an antioxidant, have been suggested as a cause for increased cancer rates in shift workers 23. Given the limited evidence that such interventions can correct the deficit in night time psychomotor performance, we advocate mitigating risk to patients by building in extra checks during critical tasks, especially towards the physiological nadir in the early hours of the morning 2425…”