Sleep Traits and Cancer: New Evidences of Breast Cancer Risk

Synchronizing sleep-wake pattern to night-day cycle is critical for good health. WHO classifies body clock disruption as probably carcinogenic in nature. A new study in The BMJ has investigated direct effects of sleep traits (morning or evening preference, sleep duration and insomnia) on risk of developing breast cancer and found that women with preference to getting up early in the morning had lower risk, also if the sleep duration is more than 7-8 hours it increases breast cancer risk.

The World Health Organization’s International Agency for Research on Cancer classifies shift work involving circadian disruption as probably carcinogenic to humans. Evidences point towards a positive connection between disruption in body clock and increased cancer risk.

Studies have shown that women workers who work night shift have higher breast cancer risk due to disruption of internal body clock caused by erratic and disturbed sleep patterns, exposure to light in twilight hours and associated lifestyle changes. However, not many studies have focused on investigating associations between one’s sleep traits (a) one’s chronotype i.e. the time of the sleep and regular activities (sleep-wake pattern) (b) sleep duration and (c) insomnia with breast cancer risk. Self-reporting by women in observational studies is prone to error or unmeasured confounding and thus making a direct inference about relationship between these sleep traits and risk of breast cancer is very challenging.

A new study published on June 26 in The BMJ aimed to investigate the causal effects of sleep traits on risk of developing breast cancer using a combination of methods. Researchers utilized two large high-quality epidemiological resources – UK Biobank and BCAC study (Breast Cancer Association Consortium). UK Biobank study had 180,216 women participants of European descent of whom 7784 had breast cancer diagnosis. 228,951 women participants, also of European descent, in BCAC study of which 122977 were breast cancer cases and 105974 controls. These resources provided breast cancer status, confounding (unmeasured) factors and genetic variables.

Participants completed questionnaire which included sociodemographic information, lifestyles, family history, medical history, physiological factors. Alongside, participants self-reported their (a) chronotype i.e. morning or evening preference (b) average sleep duration and (c) insomnia symptoms. Researchers analyzed the genetic variants associated with these three particular sleep traits (recently identified in large genome-association studies) by using a method called Mendelian Randomization (MR). MR is an analytic research method used to investigate causal relationships between modifiable risk factors and health outcomes by using genetic variants as natural experiments. This method is less likely to be affected by confounding factors compared to traditional observational studies. Several factors which were considered as confounders of the association between sleep traits and risk of breast cancer were age, family history of breast cancer, education, BMI, alcohol habits, physical activity etc.

Mendelian analysis of UK Biobank data showed that ‘morning preference’ (a person who wakes up early in the morning and goes to bed early in the evening) was associated with lower risk of breast cancer (1 less woman in 100) compared to ‘evening preference’. Very little evidence showed possible risk association with sleep duration and insomnia. Mendelian analysis of BCAC data also supported morning preference and further showed that longer sleep duration i.e. more than 7-8 hours increases risk of breast cancer. The evidences of insomnia were inconclusive. Since MR method gives reliable results so if an association is found, it is suggestive of a direct relationship. The evidences were seen to be consistent for both these causal associations.

The current study integrates multiple approaches to be able to make an assessment about the causal effect of sleep traits on risk of breast cancer by first, including data from two high quality resources – UK Biobank and BCAC and second, use data derived from self-reporting and objectively assessed measures of sleep. Further, MR analysis used highest number of SNPs identified in genome-wide association studies till date. The findings reported have strong implications for persuading good sleep habits in the general population (especially younger) in order to improve one’s health. The findings could help to develop new personalized strategies for reducing risk of cancer associated with disruption of our circadian system.


{You may read the original research paper by clicking the DOI link given below in the list of cited source(s)}


1. Richmond RC et al. 2019. Investigating causal relations between sleep traits and risk of breast cancer in women: mendelian randomisation study. BMJ.
2. UK Biobank.
3. Breast Cancer Association Consortium.

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