Time spent in outdoor light is associated with increased blood pressure, increased hypertension risk, and decreased hypotension risk.

Objective: Light exposure is thought to be associated with blood pressure (BP). However, the existing evidence is inconsistent, and the underlying mechanisms remain unclear.

Methods: This cohort study enrolled over 300,000 participants from the UK Biobank. Information on the time spent in outdoor light during typical summer and winter days was collected using questionnaires. Cases of hypertension and hypotension were identified using the 10th edition of the International Classification of Diseases codes. Cox proportional hazards regression models were employed to estimate the lightBP associations, and restricted cubic splines were utilized to detect potential nonlinear associations. Subgroup analyses were conducted to identify effect modifiers, and causal mediation analyses were performed to explore potential mechanisms.

Results: Using summer light exposure as an illustration, after a median follow-up of 13.4 years, each additional hour of summer light exposure was found to be associated with an increased risk of hypertension (hazard ratio [HR] 1.011, 95 % confidence interval [CI] 1.0061.017, P-nonlinear=0.803) and a decreased risk of hypotension (0.988, 0.9770.998, P-nonlinear=0.109). The lightBP association was found to be stronger in females (P=0.022), those with short sleep durations (P=0.049), and those with a high genetic risk of hypertension (P<0.001). Potential mechanisms included increases in biological age (proportion mediated, 24.1 %, P<0.001), neutrophil count (5.4 %, P<0.001), body mass index (32.0 %, P<0.001), etc. CONCLUSIONS: Our study revealed a positive lightBP association. Potential mechanisms include inflammation, aging, and lifestyle changes. Further epidemiological and experimental investigations are necessary to validate these findings.