Results
Evening chronotype was associated with higher risk of breast cancer compared to morning chronotype.
- HR = 1.12; 95% CI: 1.07–1.17
- Low between-study heterogeneity (I² = 14%)
- Based on 4 cohort studies
- Random-effects meta-analysis was used
Sleep
Unravelling the association between sleep traits and reproductive cancers: A systematic review and meta-analysis.
TL;DR
Evening chronotype was associated with higher risk of breast cancer and epithelial ovarian cancer, while insomnia showed elevated breast cancer risk that disappeared after proper adjustment for confounders, supporting a role for circadian disruption in carcinogenesis.
Key Findings
Results
Evening chronotype was associated with higher risk of epithelial ovarian cancer compared to morning chronotype.
- HR = 1.15; 95% CI: 1.02–1.29
- No between-study heterogeneity (I² = 0%)
- Based on 2 cohort studies
- Random-effects meta-analysis was used
Results
Insomnia was associated with elevated breast cancer risk across cohort studies.
- RR = 1.23; 95% CI: 1.01–1.50
- High between-study heterogeneity (I² = 94%)
- Based on 9 cohort studies
- The association remained similar for diagnosed insomnia specifically (RR = 1.26; 95% CI: 1.01–1.58; I² = 96%; 5 studies)
Results
The association between insomnia and breast cancer risk was lost when restricted to properly adjusted studies.
- In properly adjusted studies: RR = 1.04; 95% CI: 0.95–1.14
- Heterogeneity was substantially reduced (I² = 41.5%) in this subset
- Based on 3 cohort studies with adequate confounder adjustment
- This suggests earlier positive findings may have been driven by residual confounding
Methods
The systematic review and meta-analysis included 62 observational studies from 55 publications.
- Study designs included 45 cohort studies and 17 case-control studies
- Literature searched across PubMed, Scopus, and Web of Science up to September 2024
- Sleep traits examined included duration, insomnia, chronotype, and snoring
- Reproductive cancers examined included breast, prostate, endometrial, and epithelial ovarian cancers
- Estimates were extracted from maximally adjusted models; cohort studies were primarily considered in interpretation
Conclusions
The findings support a role for circadian disruption in carcinogenesis.
- Both chronotype (evening vs. morning) and insomnia were evaluated as proxies of circadian disruption
- The authors note that future research should use objective sleep assessments
- The authors call for evaluation of whether modifying sleep behaviours or circadian alignment can reduce cancer risk
- Cohort study findings were prioritized over case-control findings in the interpretation
What This Means
This research suggests that certain sleep habits and patterns may be linked to an increased risk of some reproductive cancers. Specifically, people who are naturally 'night owls' (evening chronotype) appear to have a modestly higher risk of breast cancer (about 12% higher) and epithelial ovarian cancer (about 15% higher) compared to 'morning people.' These associations were based on multiple large cohort studies and showed relatively consistent results across studies. The relationship between insomnia and breast cancer was more complex. While an overall analysis of nine cohort studies showed a 23% higher risk associated with insomnia, this association disappeared when the analysis was restricted to studies that had most carefully accounted for other lifestyle and health factors. This suggests that the apparent insomnia-breast cancer link seen in some studies may be partly explained by other factors rather than insomnia itself causing cancer risk. This research matters because sleep patterns are potentially modifiable — unlike genetic risk factors, people may be able to change their sleep behaviors. However, the authors caution that the current evidence relies largely on self-reported sleep measures rather than objective tracking tools, and that future studies are needed to determine whether improving sleep timing or quality could actually lower cancer risk. The findings broadly support the idea that disruptions to the body's internal clock (circadian rhythm) may play a role in cancer development.
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Citation
Pagkalidou E, Papagiannopoulos C, Manou M, Filis P, Richmond R, Tsilidis K, et al.. (2026). Unravelling the association between sleep traits and reproductive cancers: A systematic review and meta-analysis.. Sleep medicine reviews. https://doi.org/10.1016/j.smrv.2026.102235