More phone use after 10 pm meant later, shorter sleep on the same night

Design

Intensive longitudinal cohort with passive sensing (ABCD Study)

Population

425 US adolescents, mean age 14.8, 7,591 person-nights

Primary outcome

Fitbit-measured sleep onset, offset and total sleep time

Effect

Per extra minute after 10 pm: onset +0.87 min (0.84–0.90); total sleep −0.43 min (−0.47 to −0.39)

Most evidence linking phones and teenage sleep relies on self-report. This JAMA Pediatrics cohort, published on 19 October, used passively recorded phone use and Fitbit-measured sleep in 425 US adolescents (mean age 14.8) across 7,591 nights.

Comparing each teenager with themselves, nights with more phone use after 10 pm than their usual were associated with falling asleep about 65 minutes later and sleeping about 20 minutes less. Each extra minute of use after 10 pm was linked to sleep onset 0.87 minutes later and 0.43 minutes less total sleep. The association grew steadily with later cut-offs from 8 pm to midnight. Use in the last few minutes before falling asleep was linked to slightly more wakefulness in the first half-hour of sleep.

The within-person design removes stable differences between teenagers, which strengthens the case, but it remains observational: a teenager who cannot sleep may pick up the phone. The sample used only Android phones and was from the US.

• Ask adolescents about phone use after 10 pm as part of any sleep, mood or school-performance consultation.
• Suggest phones are charged outside the bedroom overnight.
• A fixed device-off time is a concrete, measurable target for families to agree.
• Address the reverse direction too: a teenager using the phone because they cannot sleep may have anxiety or a circadian delay.

Why it matters

Objective measurement links late-evening phone use to lost sleep on the same night, giving a specific target rather than vague screen-time advice.

Don't overread it

This is observational; it does not show that restricting phone use will lengthen sleep by the same amount.

The statistics, in plain English

Within-person analysis compares a teenager's heavy-use nights with their own lighter-use nights, so differences between teenagers (personality, family rules) cannot explain the result. It still cannot rule out that sleeplessness led to phone use rather than the reverse.

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