For parents worried about how much time their teenager spends on a phone, a new study points to a different lever with a much bigger effect. Sleep, not screen time, appears to be the lifestyle factor most closely tied to how well 10 to 13 year olds do in school and get along with friends.
The research, published in the journal Psychological Medicine, came out of UC Davis Health and drew on data from the Adolescent Brain Cognitive Development Study, a long-running U.S. project that enrolled 11,880 children at ages 9 and 10 and follows them through adolescence.
Analyzing sleep, screen time, physical activity and diet, the researchers found that sleep quality explained far more of the link between mental health symptoms and lower grades than the other three factors combined. For anxiety, sleep accounted for 36.3 percent of the association. For depression, 18.5 percent. For psychotic-like experiences, 8.3 percent.
Screen time came in a distant second, explaining roughly 5 to 6 percent of these associations. Exercise and diet had only minor effects on academic and social functioning at this age.
Also read: This is what happens to your body when you use air conditioning for long periods
Why the finding surprised the researchers
Jason Smucny, a computational neuroscientist at UC Davis and the study's first author, said the size of the sleep effect stood out. "This finding surprised us. We expected sleep to be important, but we didn't expect it to stand out so clearly above other lifestyle factors, particularly screen time," he told News-Medical.
The effect was most visible in adolescents already dealing with anxiety, depression or other mental health challenges. In this group, sleep appeared to shape everyday functioning, from grades and friendships to prosocial behavior, far more than the other habits families often focus on.
The researchers are careful to note that the study is observational. It shows a strong statistical link, not proof that shifting bedtimes alone will lift grades or repair social difficulties. They also stress that healthy sleep habits are not a substitute for professional evaluation and treatment when a young person is struggling.
What families can do first
Senior author Tara Niendam, a professor of psychiatry at UC Davis, framed the practical takeaway around what parents actually have control over. "Sleep is something parents and families can address," she said, adding that healthy sleep routines are a practical way to support a child's emotional well-being.
Also read: Only 1 in 3 US men gets the pre-biopsy prostate MRI that is standard in Europe
For 13 to 18 year olds, the U.S. Centers for Disease Control and Prevention recommends 8 to 10 hours of sleep per 24 hours. Younger adolescents typically need slightly more. Most American teens fall short. CDC data has previously shown that around 7 in 10 high schoolers do not get enough sleep on school nights.
Basic sleep hygiene, outlined by the National Sleep Foundation, includes keeping a consistent bedtime on both weekdays and weekends, putting phones and other devices away for at least half an hour before lights out, avoiding caffeine in the afternoon and evening, and keeping the bedroom cool, dark and quiet.
If a teenager shows persistent sadness, anxiety or a sudden drop in school or social functioning, the researchers say sleep changes should be paired with a conversation with a doctor or mental health professional, not used in place of one.
Also read: Throw these cans away immediately, neurologist warns
Also read: These natural remedies may help relieve pain, nausea, and restlessness