Is problematic internet use always problematic? An experience-sampling study of compulsive and avoidance-driven internet behaviors and momentary mood.

Objective: The internet has transformed communication and entertainment.Population-level studies typically find that higher problematic internet use (PIU) is associated with lower well-being.We explored how well that average describes individuals using an idionomic (within-person-first) design.Method: Over 10 days, 84 young adults (M age = 23.5) completed five daily surveys assessing four Young's (1998) PIU-related behaviors (two time control/compulsivity; two emotion dysregulation/avoidance) and six bipolar mood states.We estimated withinperson associations using idiographic autoregressive integrated moving average with exogenous inputs, summarized heterogeneity with random-effects and Bayesian multilevel meta-analysis, and explored subgroups via partitioning around medoids and growing self-organizing Maps.Results: At the within-person level, being above one's own PIU baseline generally linked to lower immediate mood, though effects varied: For some, higher-thanusual PIU coincided with better mood, for others, with worse mood.At the between-person level, individuals with habitually higher PIU reported lower average mood.Conclusions: Findings support measurement-based tailoring: If higher-than-usual PIU lifts mood now but carries later costs, practitioners might emphasize delay-ofgratification strategies; if it dampens mood immediately, they may use awareness/functional mapping approaches.Future studies should test whether person-specific measures improve intervention choice. Public Health Significance StatementMost research treats problematic internet use as uniformly harmful, but this study shows that its effects on mood vary widely from person to person.We found that while some individuals felt worse when their internet use was higher than usual, others felt better or showed no change at all.Rather than applying a one-size-fits-all approach, practitioners can use each person's own pattern of internet use and mood to tailor intervention, focusing on building awareness of immediate harms for some and on managing short-term benefits that may carry longer term costs for others.