Morning lark or night owl: the link between sleep traits and reproductive cancers

New research provides a comprehensive analysis of the current evidence on associations between sleep duration, chronotype, insomnia, and snoring, and the risk of reproductive cancers.

A chronotype is an individual’s natural preference for the timing of sleep and wakefulness across the 24-hour day.

A recent study, published in Sleep Medicine Reviews, found that women with an evening chronotype (a preference for later sleep/wake times) were found to have a higher risk of breast and epithelial ovarian cancer, compared to those with a morning chronotype. Led by Dr Christos Chalitsios and Dr Eirini Pagkalidou, and funded by World Cancer Research Fund (WCRF), a team at the University of Ioannina in Greece gathered and analysed data from observational studies on sleep and reproductive cancers.

Overall, the evidence suggests that circadian misalignment – such as staying awake late at night – may be an important sleep-related factor associated with cancer risk, rather than general sleep quality or quantity.

University of Ioannina’s Dr Christos Chalitsios, said:

“This research builds on existing evidence which suggests that a later sleep chronotype may be associated with a higher risk of breast and ovarian cancer. Overall, these findings help clarify how different aspects of sleep may relate to reproductive cancers and further research is needed to better understand the underlying biological mechanisms, and whether modifying sleep behaviours can reduce cancer risk.”

A link between chronotype and breast and epithelial ovarian cancer

Reproductive cancers, which are cancers related to sex hormones or reproductive organs, such as breast, prostate, endometrial and epithelial ovarian cancer, significantly contribute to the global cancer burden. Breast cancer is the most common cancer in women worldwide and prostate cancer is the second most common cancer in men.

Dr Chalitsios and his team carried out a systematic review of 62 observational studies, and a meta-analysis of 61 individual studies from 55 publications, that examined different sleep traits – namely insomnia, sleep chronotype, snoring and sleep duration.

From their analysis, the team found that an evening chronotype (‘night owls’) was associated with a 12% higher risk of breast cancer and a 15% higher risk of epithelial ovarian cancer compared to a morning chronotype (‘morning larks’).

World Cancer Research Fund International’s Assistant Director of Research and Policy, Dr Helen Croker, said:

“While sleep is widely recognised as an important contributor to overall health, its role in cancer risk has remained unclear. These findings provide the first systematic review and meta-analysis evaluating the association between sleep traits and reproductive cancers and help strengthen the evidence that later sleep patterns may be associated with a higher risk of breast and epithelial ovarian cancers.”

No clear evidence for insomnia

However, the team found that the evidence for insomnia and sleep duration was inconsistent. While some analyses initially suggested an increased breast cancer risk with insomnia, this association was not observed in studies that better accounted for potential confounders, pointing to the impact of multiple factors. Although insomnia can be particularly challenging to measure, further research is required to try and better understand this link.

Similarly, no clear relationship was observed between sleep duration and reproductive cancer risk, with different findings across studies and no convincing dose-response pattern, suggesting that the amount of sleep people get each day is unlikely to be an independent risk factor.

Dr Chalitsios said:

“There was no consistent or convincing evidence linking sleep duration, insomnia or snoring, with cancer risk. Although some analyses suggested a higher risk of breast cancer for people with insomnia, particularly for clinically diagnosed insomnia, the association wasn’tseen in studies that more fully adjusted for potential confounders.”

These findings support the hypothesis that circadian disruption may play a role in cancer development and are consistent with existing theories on the carcinogenic effects of light at night, as well as evidence of increased cancer risk among night shift workers exposed to artificial light at night.

Dr Croker added:

“We are delighted to see WCRF International’s INSPIRE Research Challenge grants supporting early career researchers to generate valuable insights into emerging areas of cancer research, with the potential to inform future recommendations on healthy sleep habits.”