Background: The associations between sleep characteristics and adiposity in children under three years are not fully understood yet. Objective: The objective of the study is to examine the cross-sectional and prospective associations between sleep characteristics and adiposity in toddlers over a 12-month period. Methods: Participants were 202 toddlers from the GET-UP! Study. Sleep duration, sleep timing, and sleep variability were assessed using 24-hour accelerometry for seven consecutive days. Height and weight were measured, and BMI z scores were calculated. Linear mixed models were performed to examine the cross-sectional and prospective associations between sleep characteristics and adiposity, with adjustments for clustering effects and demographic factors. Results: Total sleep duration was negatively associated with higher adiposity cross-sectionally (B = −0.12; 95% CI: −0.23, −0.01;.033) but not prospectively (B = 0.01; 95% CI: −0.13, 0.10;.843). Nap duration was prospectively associated with higher levels of adiposity (B = 0.41; 95% CI: 0.14, 0.68;.003). Sleep variability and sleep timing were not associated with concurrent or subsequent adiposity. Conclusion: Although sleep duration is an important factor associated with obesity in toddlerhood, the potential effects of different types of sleep duration may vary. While longer total sleep duration may protect children from increasing adiposity, longer nap duration seems to be risk factor. As evidence in this age group is scarce, more research is needed to confirm this finding.