What would an optimal 24-hour day look like for children? According to the Canadian 24-Hour Movement Guidelines for Children and Youth, for kids between the ages of eight and 11, it should include at least 60 minutes of physical activity, two hours or less of recreational screen time, and nine to 11 hours of sleep. Yet, in a new study, only one in 20 US children met all three of these recommendations.
The research, published on Thursday (Sept. 27) in the academic journal Lancet Child & Adolescent Health (paywall), used data from the Adolescent Brain Cognitive Development (ABCD) study, a 10-year, longitudinal, observational study of over 4,500 children between eight and 11 years old, from 21 study sites across the US, and compared their daily exercise, technology, and sleep habits to the guidelines. The researchers then assessed the participants’ “global cognition” with standards developed by the National Institute of Health.
They found that only 5% of children met all three recommendations. Sixty-three percent of children spent more than two hours a day staring at screens, going over the screen-time limit; 82% of children failed to meet the guidelines for daily physical activity; and 49% did not get the recommended hours of sleep. Twenty-nine percent met none of the recommended standards.
The rare few who met all three criteria performed better on cognitive, language, and memory tests than kids who met none of the recommendations. The study also found that of the three recommendations, the screen-time guideline seemed to correlate most strongly with superior mental performance: as long as children met the screen time recommendation, they outscored others in the study on the global cognition tests.
Meeting the sleep guidelines, meanwhile, was meaningfully correlated to higher cognition scores, but appeared far less significant than meeting screen-time recommendations.
On the other hand, the effect of physical activity on a child’s cognition was statistically insignificant in this study. The researchers say this is likely because respondents filled out a self-reported questionnaire, meaning the study wasn’t able to distinguish between intensity or type of physical activity. Lead author Jeremy Walsh says that the ABCD study authors are considering measuring physical activity through the use of a FitBit in the next cohort of participants to offset this problem.
The authors write that their findings “highlight the importance of limiting recreational screen time and encouraging healthy sleep to improve cognition in children.” This conclusion comes at a time of heightened awareness of the negative impact of excessive screen time on kids’ development–especially in early childhood (before the age of five) and in adolescence (after the onset of puberty, when the human brain is particularly vulnerable to psychiatric diseases). As Jenny Anderson has written for Quartz:
Too much use of technology can cause stress in the brain, which has two negative effects. First, more stress leads the brain to release cortisol, which can kill neurons on the “memory” center of the brain (the hippocampus). Second, the stress can inactivate the brain’s prefrontal cortex, or the “executive” part of the brain, which normally limits dopamine and our sense of pleasure or reward. When the brain gets used to a higher level of dopamine, it wants us to keep seeking out the addictive substance or habit.
The latest Lancet study has several limitations, chief among which is that correlation is not causation: As observers have noted, while it showed an association between reduced screen time and children’s cognitive skills, it did not establish a causal link. And, as experts on technology’s impact on kids have said, further research is needed to find out which types of screen activities affect children, and to what extent. But what’s certain, as Hamza Shaban writes in The Washington Post, is that “Parents who possess the resolve to separate their children from their smartphones may be helping their kids’ brainpower.”