While physical activity (PA), sedentary time (ST), and sleep each show individual associations with learning outcomes, their combined associations remain largely unexplored. This cross-sectional study examined the association between the 24-h movement behavior composition and arithmetic and reading fluency in children and adolescents, gender differences in these associations, and theoretical 30‑minute behavior‑change scenarios.

Volunteered children (N = 253, mean age: 9.5 ± 0.4 years, 53% girls) and adolescents (N = 174, mean age: 13.7 ± 0.6 years, 63% girls) participated. Children’s arithmetic fluency was measured using the FUNA test battery, and reading fluency with the Sentence Reading fluency test. Adolescents’ arithmetic fluency was measured with the Basic Arithmetic Test, and reading fluency with the word reading task ALLU. Light PA, moderate PA, vigorous PA, and ST were measured using hip-worn accelerometers (ActiGraph GT3X +). Students recorded their sleep time. The associations were examined using compositional data analysis. The ratios of behaviors were predictors within linear mixed models, adjusted for age, gender, special educational needs, body fat percentage, and guardians’ education.

The light PA relative to other behaviors was inversely associated with reading fluency (β = − 0.98, p = 0.014) among children. No other behaviors were associated with reading or arithmetic fluency in children or adolescents. Among adolescents, there was a significant interaction effect of gender and the time spent in ST on reading fluency (β = − 2,85, p = 0.037), but not on arithmetic fluency. Together, the interaction and main effects indicate that higher ST is linked to better reading fluency for both genders, with a stronger association in boys. Among adolescents, predicted 30‑minute reallocations from ST to sleep, or moderate PA, as well as from LPA to sleep, increased the difference in reading fluency between girls and boys.

Lower levels of light-intense PA were linked to better children’s reading fluency. The association between the 24-h activity composition and reading fluency among adolescents may differ between boys and girls. From a 24-h movement behavior perspective, supporting reading may require strategies that are tailored by age and gender.