Time Outdoors in Reducing Myopia - A School-Based Cluster Randomized Trial with Objective Monitoring of Outdoor Time and Light Intensity (STORM)

6,295 students aged 6-9 in 24 primary schools; control 2,037 / test I 2,329 / test II 1,929

Primary school children aged 6-9 in Shanghai, China, followed 2 years

Large multi-site, examiner-masked, 3-arm cluster RCT stratified and randomized by school 1:1:1. Test arms received an additional 40 or 80 minutes of outdoor time per school day. Compliance was objectively monitored with wrist-worn wearables measuring outdoor time and light intensity in year 2 - the methodological advance over earlier trials, which relied on diaries. No academic outcome measured.

Unadjusted 2-year cumulative myopia incidence 24.9% (control), 20.6% (test I), 23.8% (test II). Adjusted incidence fell 16% in test I (IRR 0.84, 95% CI 0.72-0.99, P = .035) and 11% in test II (IRR 0.89, 95% CI 0.79-0.99, P = .041). Less myopic shift and axial elongation in both test arms (test I -0.84 D / 0.55 mm, test II -0.91 D / 0.57 mm, control -1.04 D / 0.65 mm). No dose-response between the 40- and 80-minute arms: objective monitoring showed both test arms achieved essentially identical outdoor time (127 min/day) versus 106 min/day in control, i.e. the prescribed 80-minute dose was never actually delivered. Authors conclude compliance monitoring is critical.

Randomization at school level removes genetic confounding of the arm contrast. The secondary dose-response analyses (outdoor minutes and lux against myopic shift) are observational within-trial comparisons and are confounded by whatever makes a child go outdoors.