Larks, owls, and breast cancer

The linked study by Richmond and colleagues (doi:10.1136/bmj.l2327) suggests that the risk of breast cancer could be causally influenced by a woman’s chronotype, as morning types (popularly known as larks) were found to have a lower risk of breast cancer than evening types (popularly known as owls).1 The signals for insomnia symptoms and sleep duration, which were also examined in this mendelian randomisation study, were considerably weaker, indicating that the biological effects of chronotype are independent of these aspects of sleep. These findings follow a previous study from the same UK cohort, showing that morning chronotypes also had lower all cause and cardiovascular disease mortality.2 The flip side of this coin is an increased risk of illness for evening types.