Higher Daytime Light Exposure Predicts Lower Risk of Gastrointestinal Cancer Incidence and Mortality.

Daytime light exposure is essential for health, primarily by synchronizing tissue and cellular-level clocks with the external light/dark cycle. Insufficient exposure may disrupt circadian alignment and contribute to adverse outcomes, including cancer. Given the high prevalence of gastrointestinal cancer, we conducted a prospective cohort study of 89,069 participants with objectively measured daytime light intensity and duration. Cox proportional hazards models were used to evaluate associations with gastrointestinal cancer incidence and mortality. Over a median follow-up of 8.8 years (804,111 person-years), 1692 gastrointestinal cancer cases were recorded, of which 891 were fatal. Higher mean levels of daytime light (≥ 1916 lux between 7:30-20:30, based on the 80% cut-off) were associated with lower risk of gastrointestinal cancer incidence (Hazard Ratio [HR]: 0.87, 95% confidence interval [CI]: 0.76-0.99, p = 0.04) and mortality (HR = 0.76, 95% CI: 0.63-0.93, p = 0.008), particularly for pancreatic cancer incidence (HR = 0.58, 95% CI: 0.40-0.85, p = 0.005) and mortality (HR = 0.47, 95% CI: 0.30-0.73, p = 0.001). Daytime light exposure ≥ 2.4 h (≥ 5000 lux between 7:30 and 20:30, which is often used as a chronotherapeutic threshold) was associated with a lower risk of pancreatic cancer incidence and mortality. Predictive ability of daytime light metrics exceeded that of sleep quality, diet, depression, and alcohol consumption. Higher daytime light exposure was associated with lower risks of gastrointestinal cancer incidence and mortality, especially for pancreatic cancer, indicating a potential protective effect that warrants further investigation in prevention and prognostic contexts.