The Mediating Roles of Sleep Quality and Physical Activity in the Association Between Smartphone Addiction and Physical Fitness Among College Students

Background: In recent years, the decline in physical fitness among college students has emerged as a public health concern, with smartphone addiction considered a potential contributing factor. However, previous studies have predominantly relied on self-reported health metrics, and potential pathways underlying its association on objective physical fitness remain to be further elucidated. Based on objective physical fitness test data, this study aimed to examine indirect associations through sleep quality and physical activity in the association between smartphone addiction and physical fitness among college students. Method: This cross-sectional study included 3400 undergraduates from one university in eastern China. Smartphone addiction, sleep quality, and physical activity were assessed using the Mobile Phone Addiction Index, Pittsburgh Sleep Quality Index, and International Physical Activity Questionnaire-Short Form, respectively. Physical fitness scores were obtained from the National Student Physical Health Standard. A parallel mediation model was estimated using structural equation modeling, adjusting for sex and body mass index (BMI). Indirect effects were evaluated using a nonparametric percentile bootstrap procedure with 2000 resamples. Results: (1) Higher smartphone addiction scores were associated with lower self-reported physical activity (r = −0.085, p < 0.001) and poorer sleep quality (r = 0.396, p < 0.001). (2) Higher objective physical fitness scores were weakly associated with greater physical activity (r = 0.062, p < 0.001) and better sleep quality (r = −0.042, p < 0.05). (3) Mediation analysis revealed that the direct association of smartphone addiction on physical fitness was non-significant (B = −0.014, 95% CI [−0.045, 0.015]), whereas the total indirect association through sleep quality and physical activity was statistically significant but small in magnitude (B = −0.022, 95% CI [−0.035, −0.010]). Conclusions: Higher smartphone addiction scores were indirectly associated with lower objectively assessed physical fitness through poorer sleep quality and lower self-reported physical activity, whereas the direct association was not statistically significant. However, the observed effects were small, and their practical significance should be interpreted cautiously. Longitudinal studies are needed to confirm the direction and practical relevance of these associations.