Ss58-01 Night Shift Work and Cancer Risk: Methodologic Challenges and Opportunities

Introduction The International Agency for Research on Cancer (IARC) has in 2019 classified night work (that involves circadian disruption) as probably carcinogenic based on limited evidence in humans and sufficient evidence in animals. Therefore, there is a need for further clarification of causality, which calls for new and improved epidemiological studies. Materials and Methods Based on gaps of knowledge, limitations and strengths of existing studies, I will focus on improved designs of future epidemiological studies. Results The current evidence, which is slightly improved since the 2019 IARC evaluation, primarily comes from case-control studies, which however may be prone to recall bias and differential participation. Therefore, bias-analyses are needed. The overall lack of evidence from several cohort studies is a main reason for the limited rather than sufficient epidemiologic evidence. Intensity, e.g. a high number of consecutive night shifts, may cause circadian disruption, which is considered as an important pathway for cancer. Such associations are observed in many case-control studies of especially breast cancer, but lifetime intensity information is non-existing in cohort-studies. Future studies should include information on detailed and harmonized exposure intensity (in healthcare workers and other jobs), sleep, subtypes of cancer, menopausal status, timing of birth-giving, racial and sex differences, chronotype, and preferably be based on lifetime objective working time information, including pay-roll data. Conclusions This presentation will suggest necessary priorities for “nice to know” and “need to know” areas of future epidemiological studies, which will not be cohort studies without lifelong night-time intensity exposure history.