Intended for healthcare professionals


Larks, owls, and breast cancer

BMJ 2019; 365 doi: (Published 26 June 2019) Cite this as: BMJ 2019;365:l4267

Linked Research

Investigating causal relations between sleep traits and risk of breast cancer in women

  1. Eva S Schernhammer, professor and chair
  1. Department of Epidemiology, Centre for Public Health, Medical University of Vienna, Vienna, Austria
  1. eva.schernhammer{at}

Good news for people with a preference for mornings

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.

By the clock

The discovery of “clock genes” and the molecular mechanisms that control circadian rhythms3 was followed by a rich body of work deciphering circadian function.4 This work established …

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