Parents have long suspected that late-night phone use — the phone glowing under a teenager’s duvet — is stealing sleep. A study led by Columbia University School of Nursing researchers, published in JAMA Pediatrics, now puts unusually precise numbers on the suspicion: on nights when adolescents used their phones more than usual after 10 p.m., they fell asleep about 65 minutes later and slept roughly 20 minutes less.
What the Researchers Measured
The team analysed more than 7,500 nights of passively tracked smartphone activity alongside Fitbit-measured sleep from 425 American teenagers taking part in the NIH-funded Adolescent Brain Cognitive Development study. Because the phone data was captured passively rather than self-reported, the study avoids one of the usual weaknesses of screen-time research, in which participants guess at their own habits.
The dose relationship was strikingly consistent. Each additional 30 minutes of phone use after 10 p.m., beyond a teenager’s own usual amount, was associated with falling asleep about 26 minutes later, waking about 11 minutes later, and losing around 13 minutes of sleep overall. Teenagers in the study averaged nearly four hours of phone use a day, including about 40 minutes after 10 p.m., most of it on social media and video apps.
Late-Night Phone Use: Why Timing Matters
Lead author Professor Yihong Zhao of Columbia Nursing was careful about what the study can and cannot claim. Because it is observational, it cannot prove that phone use causes the sleep changes; it is possible, for example, that teenagers who cannot sleep reach for their phones. But the findings suggest the timing of use matters as much as the total, a point echoed in coverage of the study’s findings.
One detail supports that view: teenagers who used their phones in the final five minutes before falling asleep spent more time awake in the first half hour after sleep onset. The researchers describe the results as pointing to practical targets — limiting use after 10 p.m. and keeping the phone out of the last minutes before sleep — rather than blanket bans, which sits interestingly alongside separate research on how school phone restrictions affect students, where structure rather than total prohibition also did much of the work.
Why Teenage Sleep Is Worth Protecting
Adolescent sleep is not a lifestyle luxury. It is bound up with memory consolidation, emotional regulation, academic performance and physical development, and teenagers’ body clocks naturally drift later, which already squeezes their sleep on school nights. Losing a further hour to late-night scrolling lands on a system that has little slack to give.
The study’s authors, who include researchers from Stony Brook University and Seattle Children’s Research Institute, frame their work as a starting point for intervention studies rather than a verdict. But for families negotiating bedtime rules this week, the message is unusually concrete: the clock on the phone may matter as much as the minutes.
Screen-time debates often dissolve into vague anxiety. This research is useful precisely because it refuses to: it identifies a window (after 10 p.m.), a behaviour (use beyond one’s own norm) and a measurable cost. Whether families respond with chargers outside bedrooms or agreed cut-offs, the study gives the conversation something it usually lacks — numbers.
