Background <p>Although most children nap at age 2, 94% have ceased by the age of 5. Naps may allow children to meet their sleep needs but may also delay and reduce nighttime sleep. The aim of the study was to understand the interrelation between objectively measured naps and nighttime sleep in children aged 2 to 5 years.</p> Methods <p>Participants were recruited within six French preschools in 2021. Sleep was measured by actigraphy during an average of 7.8 days (5 to 10 days) in 85 children (M_age = 3.8 years, SD_age = 0.56, 46% girls). Paired t-tests were used to study the difference in nap and nighttime sleep between school days and weekends, and linear mixed models, adjusted for age and sex, to analyze the day-to-day effect of naps on nighttime sleep.</p> Results <p>On school days, when all children had the opportunity to take a nap and the time was regulated by the school, naps were more frequent (72% vs 38%) but shorter (62 vs 81 min) and occurred earlier (rise time 15:17 vs 16:00) than on weekends (all <i>p</i> &lt; 0.003). Although naps were twice as frequent on school days than on weekends, nighttime sleep onset was earlier on school days (21:20 vs 22:00 <i>p</i> &lt; 0.001), with no difference in nighttime sleep duration. Day-to-day mixed model analyses showed that an hour increase in nap duration reduced nighttime sleep by 13.6 min (SE = 6.1, <i>p</i> = 0.027), and increased sleep onset latency by 6.38 min (SE = 3.01, <i>p</i> = 0.035). On days when children napped, 24-h total sleep time increased by 45 min (SE = 4.97, <i>p</i> &lt; 0.001). Conversely, nighttime sleep duration had no effect on the following day’s nap.</p> Conclusions <p>Naps delayed and reduced nighttime sleep, but the effect was clinically marginal, and naps contributed to longer 24-h total sleep time. Large differences were observed between school days and weekends, suggesting that sleep habits do not follow sleep needs.</p>