Students use their phones for a third of the school day, and their cognitive control shows it

One-third. That is the proportion of the school day that youth spend interacting with their smartphones, according to a new cross-sectional study published in JAMA Network Open. Not at home. Not after school. During school hours, in classrooms and hallways, between and sometimes during lessons.

The finding comes from a study of youth ages 11 to 18 that measured not just how much time students spent on their phones, but how they used them, specifically how frequently they checked them throughout the day. That distinction turns out to matter.

Checking versus total time

The study, led by Eva H. Telzer at the University of North Carolina, found that smartphone use during school hours was associated with reduced cognitive control, the set of mental processes that allow people to focus attention, resist distractions, hold information in working memory, and switch between tasks.

What makes this finding particularly relevant is the nature of the smartphone use involved. The cognitive effects were tied not just to total screen time but to habitual checking behaviors, the frequent, brief phone interactions that fragment attention throughout the day. A student who spends 30 minutes on their phone in one block may be less affected than a student who checks their phone 60 times in brief bursts, because each check represents an interruption to whatever cognitive task was underway.

Why attention fragmentation matters in school

Cognitive control is not a fixed trait. It is a capacity that develops throughout adolescence and is shaped by how the brain is used during that developmental window. The prefrontal cortex, which governs cognitive control, is among the last brain regions to mature, making it particularly sensitive to environmental influences during the teenage years.

Habitual phone checking creates a pattern of repeated attention switching. Each time a student picks up their phone, checks a notification, and returns to their work, they pay a cognitive switching cost. Accumulated over hundreds of daily checks across years of school, these costs may compound in ways that affect how well the developing brain builds the neural architecture for sustained attention.

The study cannot establish this causal pathway from its cross-sectional design alone. It is possible that students with lower baseline cognitive control are more drawn to their phones, rather than phone use reducing their cognitive control. Longitudinal studies tracking the same students over time would be needed to disentangle direction of effect.

Policy implications

School phone policies are proliferating across the United States and globally, but most focus on either total bans or restrictions on total screen time. The study's findings suggest that habitual checking behaviors may be a more important target than aggregate time spent on devices. A policy that allows students to use their phones during lunch but prevents constant checking during classes might address the cognitive impact more precisely than a blanket ban.

The authors highlight the need for digital literacy programs that help students understand how their phone habits affect their attention, not as an abstract concept but as a measurable cognitive consequence. Teaching students to recognize the cost of each phone check could be more effective than rules that students work around.

Limitations

The study is cross-sectional, capturing a single snapshot rather than tracking changes over time. Self-reported phone use may not perfectly match actual usage patterns. The measure of cognitive control, while validated, reflects performance on specific tasks rather than academic outcomes directly. And the study cannot account for all potential confounders that might explain both heavy phone use and lower cognitive control, such as sleep quality, mental health, or socioeconomic factors.

Still, the core observation is difficult to dismiss: a third of the school day is a substantial amount of time to spend on a device that was not designed for learning, and the pattern of use most associated with cognitive costs is not binge sessions but the constant, fragmented checking that characterizes modern smartphone behavior.