Women who start their early day are more likely to have a lower risk of breast cancer compared to late-onset ones, according to a new study.
The study found that a preference for controls reduced the risk of breast cancer by 40% compared with the evening type.
It was also found that women who slept more than 7 to 8 hours recommended were at an increased risk of 20% of the disease for another hour.
"The findings of the protective effect of morning preference on breast cancer risk in this study are consistent with previous studies that highlight the role of night shift and exposure to night-vision as breast cancer risk factors," said Rebecca Richmond, Research student in cancer research in the UK.
"We already know that night shift work is associated with worse mental and physical health. This study provides additional evidence that disruptive sleep patterns may play a role in cancer development," she added.
The results were presented at the NCRI Cancer Conference 2018 in Glasgow.
For the study, the researchers looked at data from 180,215 women and 228,951 women who were part of a large genome study of breast cancer.
Using genetic factors associated with morning or evening preferences, sleep duration and insomnia, they investigated whether these sleep characteristics had a causal contribution to breast cancer risk.
They also found some evidence for the causal effect of increased sleep duration and split sleep on breast cancer.
The researchers believe that their findings have implications for policymakers and employers.
"These findings have potential policy implications for the impact of sleep habits of the general population to improve health and reduce the risk of breast cancer in women," said Richmond.
"We want to do more work to investigate the underlying mechanisms, because the estimates are based on questions related to morning or evening preferences, not just whether people get up early or later that day.
"In other words, it can not be that the change in your habits changes the risk of breast cancer, it may be more complicated than that," she says.