Background
The 2019 IARC evaluation of night shift work highlighted the challenges of determining whether night shift work causes cancer using traditional epidemiologic studies. Because of long disease latencies and the rarity of cancer outcomes, studies of cancer risk are limited in their ability to detect the likely modest effects of night shift work, which would still have a substantial impact on cancer burden given its high prevalence. These limitations are further complicated by exposure assessment challenges, given that shift schedules change over time and night shift work is a complex exposure with multiple components that may individually contribute to cancer risk. Mechanistic studies in human populations can overcome these challenges by capturing biological effects occurring soon after exposure but well before disease onset, providing compelling evidence that establishes causal links and identifies plausible targets for prevention. Although the human mechanistic evidence available in 2019 was sparse and limited by small sample sizes, inadequate exposure characterization, and failure to account for circadian variation in biomarker levels, which requires carefully timed biospecimen collection, these limitations can be addressed through rigorous study design. Leveraging hybrid study designs and emerging biomarkers further strengthens mechanistic approaches by providing direct evidence of causal effects and characterizing the biological consequences of night shift work with greater granularity, thereby pinpointing targets to advance cancer prevention.
 
Objectives
Using examples from our completed and ongoing studies, the proposed presentation will illustrate how well-designed mechanistic studies in night shift workers can overcome the limitations of traditional epidemiologic approaches, strengthen causal inference, and guide effective cancer prevention strategies.
 
Methods
Highlighted studies will include: (1) a completed randomized crossover trial testing the impact of melatonin supplementation on oxidative DNA damage repair in night shift workers; (2) an ongoing study using deep metagenomic sequencing to characterize compositional and functional gut microbiome differences between night shift and day shift workers, while parsing the contributions of associated lifestyle factors (e.g., diet, meal timing, sleep) that may drive carcinogenic dysbiosis.
 
Results
Findings from our first-of-its-kind trial suggested that melatonin supplementation mitigates the negative impacts of night shift work on repair of oxidative DNA damage, a key mechanism for the pathogenesis of cancer. The functional metagenomic analysis in the gut microbiome study will clarify the specific biological pathways that may mediate cancer risk, offering deeper mechanistic insights than previous studies which have only focused on compositional differences. Findings from this study will inform a future hybrid study design in which stool samples from night shift workers are used to inoculate gnotobiotic mice, enabling a direct evaluation of whether night shift–associated gut dysbiosis has causal effects on colorectal tumorigenesis.
 
Conclusions/Implications
Our studies demonstrate the practical value of mechanistic approaches for evaluating complex exposures like night shift work. The melatonin trial is a proof-of-concept that mechanistic endpoints can be used to develop and evaluate intervention strategies in contexts where traditional epidemiologic studies face significant limitations. Findings of the microbiome analysis will motivate a future hybrid study directly testing causality and furthering progress toward actionable cancer prevention strategies.