Why night owls may be at greater risk of developing breast cancer

Women who are ‘night owls’ preferring to stay up late are twice as likely to develop breast cancer compared with ‘larks’ who function better at the beginning of the day, a major new study has shown.

British researchers funded by Cancer Research UK and the Medical Research Council (MRC) looked at hundreds of thousands of women to determine whether the way they sleep makes a difference to whether they develop disease.

The team compared 2,740 breast cancer survivors with 149,064 disease-free women and found larks were 48 per cent less likely to develop the diseases.

Similarly they mapped genetic variations being a night owl or a lark for 228,951 women and compared them to their risk of cancer.

Again they found a preference for early morning reduced the risk of breast cancer by 40 per cent compared with night owls.

Dr Rebecca Richmond, of the MRC Integrative Epidemiology Unit at the University of Bristol, said: “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.

“This is 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.”

Around 55,000 women are diagnosed with breast cancer each year and 11,000 will die from the disease.

The study also looked at whether the amount of sleep women got each night was linked to their risk of cancer, based on data from 85,000 UK Biobank participants who were asked to wear movement monitors.

Results showed that women who slept longer than the recommended seven to eight hours had a 20 per cent increased risk of the disease per additional hour slept.

Commenting on the study Cliona Clare Kirwan, from the University of Manchester, said: “These are interesting findings that provide further evidence of how our body clock and our natural sleep preference is implicated in the onset of breast cancer.

“We know already that night shift work is associated with worse mental and physical health. This study provides further evidence to suggest disrupted sleep patterns may have a role in cancer development.”

However other researchers cautioned that different sleeping patterns could be a side-effect of the diseases, rather than a causal factor. Dr Dipender Gill, Clinical Research Training Fellow of Imperial College London, said: “There may be a shared genetic cause for being a ‘lark’ and risk factors related to breast cancer.

“For example, the genetic determinants of sleep may also affect other neuronal mechanisms that affect breast cancer risk independently of sleep patterns. In such a scenario, sleep patterns may be associated with risk of breast cancer, but not directly cause it.”

Dr Richard Berks, Senior Research Communications Officer at Breast Cancer Now, added: “These intriguing results add to the growing body of evidence that there is some overlap between the genetics of when we’d prefer to sleep and our breast cancer risk, but more research is required to unravel the specifics of this relationship.

“Preferring to sleep all morning, and being able to, are two different things – and we now need to understand how women’s actual sleeping habits may affect their breast cancer risk. While these are interesting findings that warrant further investigation, it is currently too early to make any recommendations to women about their sleeping patterns.

“What we can be certain of is that all women – larks and owls – can reduce their risk of breast cancer by exercising regularly, maintaining a healthy weight and reducing their alcohol intake.”

The research was presented at the NCRI Cancer Conference in Glasgow.