We study how the timing of mobile push notifications affects user engagement. Industry practice assumes that delivering messages precisely when users are active—such as at screen-on moments—maximizes effectiveness. Using large-scale data from a leading push notification platform in China, we challenge this assumption through two complementary quasi-experiments. A regression discontinuity design around system “listening windows” shows that screen-on pushes reduce daily app usage. To reconcile this puzzle, we analyze cross-app data where users’ screen-on signals occur moments before push requests, creating quasi-random variation in notification delay. We find that slightly delayed screen-on pushes—arriving one to two minutes after users turn on their screens—generate significantly higher engagement than instantaneous pushes. These results reveal that hyper-precise timing can backfire: messages delivered too immediately may feel intrusive, while modest delays enhance receptivity. Our findings highlight an inverted-U relationship between temporal precision and user responsiveness, underscoring the need to balance attentional salience with user experience in mobile communication design.