It has been hypothesized that circadian disruption is related to higher cancer risk. Since the International Agency for Research on Cancer classified shift work involving circadian disruption as probably carcinogenic to humans (Group 2A), multiple studies have been conducted to test this hypothesis. The aim of this systematic review was to summarize the findings and evaluate the quality of existing epidemiological studies (case-control and cohort studies) on the relationship between night-shift

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... work and breast and prostate cancer risk. Thirty-three epidemiological studies investigating the relationship between night-shift work and breast (n = 26) or prostate (n = 8) cancer risk were included (one paper included both sites). The Newcastle-Ottawa Scale for the quality of non-randomized studies was used to assess the risk of bias of the publications. The studies included were heterogeneous regarding population (general population, nurses working in rotating shifts, and other) and measurement of exposure to night-shift work (ever vs. never exposure, short vs. long-term, rotating vs. permanent) and, thus, a diversity of outcomes were observed even within the same type of cancer. In summary, 62.5% works found some type of association between night-shift work and increased risk of cancer, for both breast and prostate. The risk of bias scored an average of 7.5 over 9 stars. Due to the limitations inherent in these studies, the evidence of a possible association between night-shift work and breast or prostate cancer risk remains uncertain and more studies providing greater control of exposure and confounding factors are required. Despite the lack of conclusive evidence, application of the precautionary principle seems advisable. Keywords. Breast cancer. Prostate cancer. Shift work. Circadian disruption. Light at night. Palabras clave. Cáncer de mama. Cáncer de próstata. Trabajo por turnos. Disrupción circadiana. Exposición a luz durante la noche.