The aim of this study was to examine the relation between chronotype and breast cancer risk. We analyzed the association between chronotype (definite morning type, probable morning type, probable evening type, definite evening type, or neither morning nor evening type) and breast cancer risk among 72 517 women in the Nurses' Health Study II (NHS II). Chronotype was self-reported in 2009, and 1834 breast cancer cases were confirmed among participants between 1989 and 2007; a 2-yr lag period was imposed to account for possible circadian disruptions related to breast cancer diagnosis. Age- and multivariable-adjusted logistic regression was used to estimate odds ratios (ORs) and 95% confidence intervals (CIs). Participants who self-reported as neither morning nor evening type had a 27% increased risk of breast cancer (multivariable-adjusted OR = 1.27, 95% CI = 1.04-1.56), compared with definite morning types. None of the other chronotypes were significantly associated with breast cancer risk (multivariable-adjusted OR = 0.99, 95% CI = 0.87-1.12 for probable morning versus definite morning types; OR = 0.96, 95% CI = 0.84-1.09 for probable evening versus definite morning types; and OR = 1.15, 95% CI = 0.98-1.34 for definite evening versus definite morning types). Overall, chronotype was not associated with breast cancer risk in our study. A modestly increased risk among neither morning nor evening types may indicate circadian disruption as a potentially underlying mechanism; however, more studies are needed to confirm our results.
