# Exposure to outdoor artificial light at night is associated with a higher risk of ulcerative colitis: a prospective cohort study from the UK Biobank

## TL;DR

Exposure to artificial light at night is linked to a higher risk of developing ulcerative colitis, but not Crohn's disease, according to a large study.

## Contribution

This study is the first to show a significant association between outdoor artificial light at night and ulcerative colitis risk in a large prospective cohort.

## Key findings

- Outdoor artificial light at night exposure was associated with an 8% higher risk of ulcerative colitis per standard deviation increase.
- The highest ALAN exposure level showed a 30.9% increased risk of ulcerative colitis compared to the lowest level.
- The association between ALAN and UC was stronger in women than in men.

## Abstract

Inflammatory bowel disease (IBD) is a chronic inflammatory condition of the gastrointestinal tract, and environmental factors are believed to play an important role in its pathogenesis. Exposure to outdoor artificial light at night (ALAN) has been linked to the globally increasing incidence and prevalence of several diseases; however, its relationship with IBD remains unclear. We aimed to estimate the long-term risk of IBD associated with outdoor ALAN exposure in a large-scale prospective cohort.

We conducted a large-scale prospective cohort study using the UK Biobank. Outdoor ALAN exposure data were obtained from satellite datasets. The primary outcome was incident IBD. Cox proportional hazards regression was used to examine the association between outdoor ALAN and the incidence risk of IBD, respectively. The non-linear association was further explored using restricted cubic spline (RCS) curves.

During a follow-up period of 13.71 years with 346,163 participants, 1,106 individuals were diagnosed with ulcerative colitis (UC), and 508 developed Crohn’s disease (CD). After adjusting for all covariates, outdoor ALAN exposure levels were positively associated with incident UC, and an 8% higher risk of UC [hazard ratio (HR), 1.084; 95% confidence interval (CI), 1.023–1.149; p < 0.001] was associated with each SD increment of outdoor ALAN exposure. The highest level of ALAN exposure was associated with a significantly increased risk of incident UC compared with the lowest level of exposure. (HR, 1.309; 95% CI: 1.12–1.529; p < 0.001). However, no significant association was observed between outdoor ALAN and CD incidence (HR, 1.044; 95% CI: 0.83, 1.308; p = 0.71). Cubic splines further indicated that outdoor ALAN was non-linearly associated with UC (p for non-linear = 0.0063). Additionally, sensitivity analysis revealed similar results, and subgroup analysis highlighted that the interaction between outdoor ALAN and UC was stronger in women than in men.

Our findings provide evidence that a higher ALAN exposure is associated with an increased risk of incident UC, with a significant dose–response relationship, but not with CD. Further studies are needed to elucidate the impact of outdoor ALAN on disease pathogenesis and outcomes.