Not OP. Here is evidence for screen time being dose dependent after more than 1 hour, but it is old so none of the kids in that study had grown up with smartph

Not OP. Here is evidence for screen time being dose dependent after more than 1 hour, but it is old so none of the kids in that study had grown up with smartphones https://www.researchgate.net/publication/283687526_Dose-response_association_of_screen_timebased_sedentary_behaviour_in_children_and_adolescents_and_depression_A_meta-Analysis_of_observational_studies https://www.researchgate.net/publication/283687526_Dose-resp... This is a summary article: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7012622/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7012622/ you will find the dose dependt point is the first paragraph under the heading "Key Points".