Abstract: Shift work that involves circadian disruption has been highlighted as a likely carcinogenic
factor for breast cancer in humans. Also, unhealthy lifestyle habits observed in night work nurses
could be causally related to an increase in the incidence of estrogen-positive breast tumours in this
population. Assessing baseline risk of breast cancer in nurses is essential. The objective of this
study was to analyze the risk of breast cancer that nurses had in relation to their lifestyle and labour
factors related to shift work. A cross-sectional descriptive study through a questionnaire about
sociodemographic variables, self-perception of health, and working life was designed. The sample
consisted of 966 nurses. The relationship between variables was tested. A binary logistic regression
and a classification and regression tree were performed. The most significant labour variables in
relation to the risk of breast cancer were the number of years worked (more than 16 years; p < 0.01;
OR = 8.733, 95% CI = 2.811, 27.134) and the total years performing more than 3 nights per month
(10 or more years; p < 0.05; OR = 2.294, 95% CI = 1.008, 5.220). Also, the nights worked throughout
life (over 500; OR = 4.190, 95% CI = 2.118, 8.287) were significant in the analysis. Nurses who had or
ever had breast cancer valued their self-perceived health more negatively (p < 0.001) and referred a
lower quality of sleep (p < 0.001) than the non-cases nurses. The occupational factors derived from
night work could have several impacts on nurses’ health and their family-work balance. Promoting
healthy lifestyles, informing about shift work risks, and adjusting shift work schedules are critical
methods to decrease the possible effects of circadian disruption in nurses
