The findings reveal that sleeping 10-plus hours may increase a woman’s risk of getting cancer and both men and women’s risk of dying from cancer.
The researchers say their findings may help refine sleep recommendations in Japan, which currently advise working, middle-aged adults to sleep “as long as they can.”
Based on the new findings, a sleep duration of 6-8 hours for men and 6-9 hours for women “may be the safest” regarding cancer incidence and mortality risk among Japanese adults, the authors conclude.
The findings were published online in the International Journal of Cancer.
The literature on sleep time and cancer risk is mixed. A trio of meta-analyses conducted between 2016 and 2019 found that long sleep duration, but not short, was associated with a slightly elevated risk of all cancer mortality in Asians.
A separate meta-analysis conducted in 2018 found that both short and long sleep durations were not related to cancer incidence. But in the stratified analysis, shorter sleep time was associated with 36% increased cancer risk among Asians.
To investigate further, the researchers pooled data from six population-based cohorts that included 271,694 adults – 126,930 men and 144,764 women – with 40,751 total incident cancer cases and 18,323 total cancer deaths during a follow-up lasting about 5.9 million person-years.
In the multivariable analysis, longer sleep duration was not associated with total cancer incidence in men. In women, however, sleeping 10 or more hours vs. 7 was associated with a 19% increased risk of cancer.
In addition, sleeping 10 or more hours was associated with an increased risk of dying from cancer in women (hazard ratio, 1.44) and men (HR, 1.18).
Sleeping for 5 hours or fewer, compared with 7, was not associated with cancer incidence and mortality. However, among postmenopausal women, shorter sleep durations did increase the risk of dying from cancer (HR, 1.15).
The authors highlight several strengths of the analysis, including a large sample size as well as stratification of the results by body mass index and menopause status, which has rarely been done in previous studies.
Limitations include self-reported sleep durations and lack of data on sleep quality. The researchers note that the mechanism by which sleep time may influence cancer incidence and mortality is unclear but likely to be complex and cancer site specific.
It’s also possible that reverse causation could explain associations between sleep duration and cancer occurrence and mortality – with pain from cancer, for instance, impairing sleep duration and quality. However, the sensitivity analysis found no evidence of reverse causality or other confounding factors.
Based on these findings, the researchers say sleep duration “may be an important variable to include in cancer incidence and mortality risk prediction models.”
The study had no specific funding. The authors declared no relevant conflicts of interest.
A version of this article first appeared on Medscape.com.