Objective: At the between-person level, it is well-documented that individuals with more physical activity (PA) and less sedentary behavior (SB) tend to have better sleep outcomes than their peers. However, the associations at the within-person level remain unclear. This study investigated the daily associations between PA and SB with nighttime sleep among young adults with and without insomnia symptoms. Methods: Data was collected through activity trackers and online questionnaires for 7 consecutive days among 147 university students, including time spent on moderate to vigorous physical activity (MVPA), light physical activity (LPA), and SB, along with sleep duration, sleep efficiency, and sleep quality. Participants were classified into two subgroups according to the presence of insomnia symptoms, which were determined by a self-reported insomnia scale. Multilevel compositional data analysis was conducted on the total sample, and separately on subsamples characterized by the presence and absence of insomnia symptoms. Results: In the total sample and subsample without insomnia symptoms, substitutions among MVPA, LPA, and SB were not associated with changes in sleep outcomes at the daily level. However, in the subsample with insomnia symptoms, days with more MVPA or SB and less LPA were associated with higher sleep efficiency, while days with more LPA at the expense of MVPA or SB were associated with lower sleep efficiency. Conclusions: For young adults with insomnia symptoms, replacing LPA with MVPA on a given day may improve their sleep efficiency that night.