Abstract and Introduction
Breast cancer is one of the most commonly diagnosed invasive cancers. Established risk factors account for only a small proportion of cases. Previous studies have found reductions in sleep duration and quality in the general population over time. There is evidence to suggest a link between poor sleep and an increased risk of breast cancer. In this study, we investigated the relationship between breast cancer and sleep duration and quality in Western Australian women. Data were obtained from a population-based case-control study conducted from 2009 to 2011. Participants completed a self-administered questionnaire that included questions on sleep. Odds ratios and 95% confidence intervals were calculated using unconditional logistic regression. Sensitivity analysis for potential selection and misclassification bias was also conducted. We found no association between self-reported sleep duration on workdays and risk of breast cancer (for <6 hours, odds ratio (OR) = 1.05 (95% CI: 0.82, 1.33); for 6–7 hours, OR = 0.96 (95% CI: 0.80, 1.16); and for >8 hours, OR = 1.10 (95% CI: 0.87, 1.39), compared with the reference category of 7–8 hours' sleep). In addition, we found no association between sleep duration on nonworkdays, subjective sleep quality, or combined duration and quality and risk of breast cancer. This study does not provide evidence to support an association between self-reported sleep duration or quality and the risk of breast cancer.
Breast cancer is the most commonly diagnosed invasive cancer and the leading cause of cancer death among women, both in Australia and globally.[1,2] While some risk factors, such as primary genetic mutations, reproductive history, weight gain, and alcohol consumption, are well established, they do not account for a significant proportion of breast cancer cases.
Both international and Australian studies have reported changes in sleep over time in developed countries. Some studies have suggested a reduction in habitual sleep duration of 13–36 minutes over the last 20–30 years,[3–7] although reductions in sleep duration have not been consistently found.[8,9] Studies of sleep quality have reported increases in the prevalence of subjective poor-quality sleep ranging from 5% to almost 40%.[3,4,10]
While the immediate effects of poor sleep, such as tiredness, loss of concentration, and injuries, are well recognized, the chronic effects of poor sleep have not been extensively studied. However, there is some evidence to suggest a link between poor sleep and a range of long-term health effects, including increased risk of cancer.[12–18]
Two plausible biological models have been proposed that would explain how poor sleep can directly influence the development of cancer: impaired immune function and metabolic pathways that lead to obesity.[2,12,15,19] An additional indirect mechanism has also been proposed which suggests an increased risk of cancer due to altered melatonin release.[20,21] Melatonin release is regulated by the light/dark cycle rather than by sleep per se, and previous studies have used sleep as a proxy for "exposure to darkness". All 3 proposed pathways have some evidence to support them, but the true process by which sleep might influence cancer risk is unknown.
Five studies have investigated the relationship between sleep duration and breast cancer and have shown mixed results.[22–26] Kakizaki et al. reported a statistically significant trend of decreasing risk with increasing sleep duration, while McElroy et al. reported a statistically significant trend of increasing risk with increasing duration. The 3 remaining studies also found significant trends, but only after study populations were restricted or polytomous analysis was conducted.[22,25,26] Only 1 of those studies also examined subjective sleep quality; no association with breast cancer was found.
Our aim in this study was to investigate the relationship between breast cancer and 3 domains of sleep (sleep duration on workdays, sleep duration on nonworkdays, and subjective sleep quality) in a population of Western Australian women.
Am J Epidemiol. 2013;177(4):316-327. © 2013 Oxford University Press