Exercise Frequency Is Associated With Greater Susceptibility to Sleep Disruption From Bedtime Smartphone Use: A Multi-University Cross-Sectional Study

Digital behaviors and physical activity jointly shape sleep in young adults, yet their interactive effects remain unclear. We conducted a short-term, multi-university cross-sectional survey of 4,730 Chinese undergraduates during a non-examination period to test whether sleep quality mediates the association between bedtime smartphone use and psychological symptoms, and how components of weekly exercise dose moderate these associations with attention to chronotype. Bedtime smartphone use was associated with poorer sleep quality, which in turn related to higher anxiety and depressive symptoms. Sleep quality mediated the associations of smartphone use with anxiety and depression, with significant indirect effects and minimal direct effects. Unexpectedly, higher weekly exercise dose amplified rather than attenuated the adverse association between bedtime smartphone use and sleep quality. Decomposition of the dose indicated that frequency was the dominant contributor to this amplification, followed by duration and with only a minor role for intensity. Effects were directionally consistent in sensitivity analyses and more pronounced among evening chronotypes. These findings highlight sleep quality as a key pathway linking pre-sleep smartphone use to mental health and suggest that dose-sensitive exercise scheduling, prioritizing fewer weekly sessions or earlier timing, together with chronotype-tailored digital hygiene, may better protect sleep in student populations with high digital exposure.