Peri‐Pubertal Height Shifts and Stable Photoperiod‐Associated Geographic Patterning Before and After COVID ‐19 in Japanese Children

OBJECTIVES: Geographic variation in child body size in Japan has previously been associated with effective day length, a light-related measure. We tested whether this prefecture-level geographic structure remained stable across the COVID-19-related disruption. METHODS: We conducted a prefecture-level ecological analysis using repeated cross-sectional summaries from Japan's School Health Statistics. For each sex and single-year age from 5 to 17 years, we compared a pre-pandemic baseline (2017-2019 mean) with a post-disruption period (2024-2025 mean). Effective day length above 5000 lx (ED5000) was treated as a prefecture-fixed exposure. Mean height was modeled as a function of mean weight and ED5000, and slope stability was tested using pooled models with an ED5000-by-period interaction term. RESULTS: Across ages 5-17 years in both sexes, mean height was generally higher in the post-period than in the pre-period, and mean weight also showed an upward shift across most ages, with the largest increases around peri-pubertal ages. ED5000 remained negatively associated with height conditional on weight in both periods, and there was no evidence that the ED5000-associated slope changed between periods at any age in either sex. In contrast, post-period differences in height appeared mainly as age-dependent upward shifts concentrated around peri-pubertal ages, peaking at ages 10-11 years in girls and 12-13 years in boys. CONCLUSIONS: These findings suggest that COVID-era disruptions were followed by age-specific increases in body size around puberty, consistent with changes in maturation tempo. In contrast, the ED5000-height association remained stable. Thus, period-related growth shifts occurred within a persistent geographic structure.