Smartphone Use During School Hours and Association With Cognitive Control in Youths Aged 11 to 18 Years

Importance: Smartphones are a pervasive feature of adolescents’ daily lives, raising concern about how smartphones are used in contexts such as school that require sustained attention and self-regulation. Objectives: To describe youths’ smartphone use during each hour of the school day and examine whether smartphone use during school is associated with poorer cognitive control, a key developmental process underlying academic success. Design, Setting, and Participants: This cross-sectional study of youths aged 11 to 18 years from the Southeastern US objectively assessed smartphone use every hour for 14 consecutive days between April 8, 2021, and February 2, 2022 (cohort 1), and February 1, 2023, and December 11, 2024 (cohort 2), providing thousands of data points to capture actual engagement. Main Outcomes and Measures: The iPhone iOS (Apple) screen time report captured smartphone use at every hour. Cognitive control was measured in the older cohort using a go/no-go task, with the signal detection metric dʹ quantifying inhibitory control. Results: A total of 79 participants (mean [SD] age, 15.10 [2.04] years; 41 [51.9%] female) participated in the study. Youths were using their smartphones during every hour of the school day, spending a total of 2.22 hours of the school day on their smartphones. Youths aged 15 to 18 years spent more time on their smartphones during school hours than those aged 11 to 14 years (mean [SD], 23.28 [18.34] vs 11.57 [16.83] min/h; F1,76 = 28.82, P < .001, η2 = 0.28). Youths spent a mean (SD) of 40.14 (39.56) minutes on social media and 13.85 (25.22) minutes on entertainment apps during school hours. Youths checked their smartphones a mean (SD) of 64.46 (32.83) times during school hours. More frequent smartphone checking was associated with lower dʹ values (F1,28 = 4.8, P = .04, η2 = 0.15), indicating poorer cognitive control. Conclusions and Relevance: This cross-sectional study found that youths use smartphones approximately one-third of the school day; this use was associated with reduced cognitive control. These findings highlight the need for school-level policies and digital literacy programs that address not only overall screen time but also habitual smartphone-checking behaviors that fragment attention.