Women who are at their best early in the morning are less likely to develop breast cancer than those who are not, a study has found.
Scientists who discovered the association also unearthed evidence of a causal link between sleeping longer and the disease.
The researchers who compared data on hundreds of thousands of women found that those with an in-built morning preference were 40%-48% less at risk of breast cancer.
Part of the analysis also showed that women who slept longer than the recommended seven to eight hours per night increased their chances of being diagnosed by 20% per additional hour spent asleep.
The study highlighted individuals who were genetically predisposed to be either "larks", an early riser or "owls", preferred the night.
Larks tend to get up and go to bed early, while owls have a body clock that leads them to feel drowsy in the morning and most energetic in the evening.
'Sleep traits have a casual contribution'
Lead scientist Dr Rebecca Richmond, from the University of Bristol, said: "Using genetic variants associated with people's preference for morning or evening, sleep duration and insomnia ... we investigated whether these sleep traits have a causal contribution to the risk of developing breast cancer.
"We would like to do further work to investigate the mechanisms underpinning these results, as the estimates obtained are based on questions related to morning or evening preference rather than actually whether people get up earlier or later in the day.
"In other words, it may not be the case that changing your habits changes your risk of breast cancer; it may be more complex than that.
"However, the findings of a protective effect of morning preference on breast cancer risk in our study are consistent with previous research highlighting a role for night shift work and exposure to 'light-at-night' as risk factors for breast cancer."
The findings were presented at the 2018 National Cancer Research Institute conference in Glasgow.
Women taking part in the study included 180,215 participants in the UK Biobank project, which stores medical research data on 500,000 individuals.
Results from 228,951 women enrolled in an international genetic study conducted by the Breast Cancer Association Consortium were also included in the analysis.