Background: Targeting specific time periods of the day or week may enhance physical activity (PA) interventions in youth. The most prudent time-segments to target are currently unclear. Objectives: To systematically review the literature describing differences in objectively-measured PA between selected time periods in young people. Methods: Electronic databases were searched for English language, cross-sectional studies of school-aged children (4-18 years) reporting time-segment specific accelerometer measured PA from 01/1990 to 01/2013. We meta-analysed standardised mean differences (SMD) between time-segments for mean accelerometer counts per minute (TPA) and minutes in moderate-to-vigorous PA (MVPA). SMD is reported in units of standard deviation; 0.2, 0.5 and 0.8 respectively represent small, moderate and large effects. Heterogeneity was explored using meta-regression (potential effect modifiers; age, sex and study setting). Results: Of 54 included studies, 37 were eligible for meta-analyses. Children were more active on weekdays than weekends (pooled SMD (95% CI) TPA 0.14 (0.08;0.20), MVPA 0.42 (0.35;0.49)). On school days, TPA was lower in-school than out-of-school, however, marginally more MVPA was accumulated in-school (TPA -0.24 (-0.40;-0.08), MVPA 0.17 (-0.03;0.38)). TPA was slightly lower on weekends than out-of-school on school days, but a greater absolute volume of MVPA was performed on weekends (TPA -0.10 (-0.19;-0.01), MVPA 1.02 (0.82;1.23)). Heterogeneity between studies was high (I2 73.3-96.3%) with 20.3-53.1% of variance between studies attributable to potential moderating factors. Conclusions: School-aged children are more active on weekdays than weekend days. The outcome measure influences the conclusions for other comparisons. Findings support tailoring of intervention strategies to specific time periods.