Phone use after 10 pm was linked to later, shorter sleep in teenagers

Design

Intensive longitudinal cohort with passive phone sensing and Fitbit sleep

Population

425 US adolescents, 7,591 person-nights

Primary outcome

Sleep onset, offset, total sleep time and wake after sleep onset

Effect

Each extra minute after 10 pm: onset 0.87 min later (0.84–0.90); sleep 0.43 min shorter

A JAMA Pediatrics cohort study used passively recorded phone use and Fitbit sleep data from 425 US adolescents (mean age 14.8) in the ABCD study, across 7,591 nights.

Comparing each teenager with themselves, nights with more phone use than usual after 10 pm were associated with sleep starting about an hour later and about 20 minutes less sleep in total. Each extra minute after 10 pm was linked to sleep onset 0.87 minutes later. Phone use in the last five minutes before sleep was linked to more wakefulness early in the night. The associations grew stronger for later cut-offs from 8 pm to midnight.

Previous studies relied on self-report. Objective measures and within-person comparisons make the link more convincing, though they still cannot prove that the phone causes the delay rather than both reflecting a late night.

The advice families already hear is supported: reduce phone use late in the evening, and keep phones out of bed.

• Ask adolescents with poor sleep or daytime tiredness about phone use after 10 pm
• Suggest a phone curfew and charging the phone outside the bedroom
• Focus on the last hour before sleep, where the association was strongest
• Ask about sleep when assessing adolescent mood, attention or school problems

Why it matters

Objective data now support what families are told about phones and sleep.

Don't overread it

Observational: within-person links strengthen the case but do not prove phones cause the delay.

The statistics, in plain English

Within-person analysis compares each teenager's heavier-use nights with their own lighter-use nights, which removes many fixed differences between people, but not night-to-night reasons for staying up.

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