LAUSR

Optimization of children's activity behaviors for skeletal health is a key public health priority, yet it is unknown how many hours of moderate to vigorous physical activity (MVPA), light physical activity (LPA), sedentary behavior, or sleep constitute the best day—the “Goldilocks Day”—for children's bone structure and function. To describe the best day for children's skeletal health, we used data from the cross‐sectional Child Health CheckPoint. Included participants (n = 804, aged 10.7 to 12.9 years, 50% male) underwent tibial peripheral quantitative CT to assesses cross‐sectional area, trabecular and cortical density, periosteal and endosteal circumference, polar moment of inertia, and polar stress–strain index. Average daily time‐use composition (MVPA, LPA, sedentary time, and sleep) was assessed through 8‐day, 24‐hour accelerometry. Skeletal outcomes were regressed against time‐use compositions expressed as isometric log‐ratios (with quadratic terms where indicated), adjusted for sex, age, pubertal status, and socioeconomic position. The models were used to estimate optimal time‐use compositions (associated with best 5% of each skeletal outcome), which were plotted in three‐dimensional quaternary figures. The center of the overlapping area was considered the Goldilocks Day for skeletal health. Children's time‐use composition was associated with all skeletal measures (all p ≤ 0.001) except cross‐sectional area (p = 0.72). Days with more sleep and MVPA, less sedentary time, and moderate LPA were beneficially associated with skeletal measures, except cortical density, which was adversely associated. The Goldilocks daily time‐use composition for overall skeletal health was center (range): 10.9 (10.5 to 11.5) hours sleep; 8.2 (7.8 to 8.8) hours sedentary time; 3.4 (2.8 to 4.2) hours LPA, and 1.5 (1.3 to 1.5) hours MVPA. Estimated optimal sleep duration is consistent with current international guidelines (9 to 11 hours), while estimated optimal MVPA exceeds recommendations of at least 60 min/d. This first study to describe optimal durations of daily activities for children's skeletal health provides evidence to underpin guidelines. © 2020 American Society for Bone and Mineral Research (ASBMR).