Crystallized intelligence
The researchers looked at 42 behavioral outcomes, 32 of which were significantly different between the groups. Four outcomes in particular – depression, thought problems, performance on a picture-vocabulary test, and crystallized intelligence – were areas where insufficient sleep seemed to have a larger negative effect.
Sleep duration’s relationship with crystallized intelligence was twice that for fluid intelligence, which does not depend on memory.
“Sleep affects memory,” Dr. Wang said. “Crystallized intelligence depends on learned skills and knowledge, which are memory. In this sense, sleep is related to crystallized intelligence.”
One limitation of the study is that some parents may not accurately report how much sleep their child gets, Dr. Wang acknowledged. Children may be awake when parents think they are asleep, for example.
And although the results show getting 9 hours of sleep may help neurocognitive development, it’s also possible that excessive amounts of sleep could be problematic, the study authors wrote.
Further experiments are needed to prove that insufficient sleep – and not some other, unaccounted for factor – causes the observed impairments in neurodevelopment.
To promote healthy sleep, parents should keep a strict routine for their children, such as a regular bedtime and no electronic devices in the bedroom, Dr. Wang suggested. More physical activity during the day also should help.
If children have high levels of stress and depression, “finding the source is critical,” he said. Likewise, clinicians should consider how mental health can affect their patients’ sleep.
More to healthy sleep than duration
“This study both aligns with and advances existing research on the importance of sufficient sleep for child well-being,” said Ariel A. Williamson, PhD, DBSM, a psychologist and pediatric sleep expert in the department of child and adolescent psychiatry and behavioral sciences at Children’s Hospital of Philadelphia and assistant professor of psychiatry and pediatrics at University of Pennsylvania, also in Philadelphia.
Dr. Ariel A. Williamson
The researchers used rigorous propensity score matching, longitudinal data, and brain imaging, which are “innovative methods that provide more evidence on potential mechanisms linking insufficient sleep and child outcomes,” said Dr. Williamson, who was not involved in the study.
While the investigators focused on sleep duration, child sleep health is multidimensional and includes other elements like timing and perception of sleep quality, Dr. Williamson noted. “For example, some research shows that having a sleep schedule that varies night to night is linked to poor child outcomes.”
Dr. Williamson tells families and clinicians that “sleep is a pillar of health,” equal to diet and exercise. That said, sleep recommendations need to fit within a family’s life – taking into account after school activities and late-night homework sessions. But extending sleep by just “20-30 minutes can make a meaningful difference for daytime functioning,” Dr. Williamson said.
Start school later?
Researchers have only relatively recently begun to understand how insufficient sleep affects adolescent neurocognitive development long term, and this study provides “crucial evidence” about the consequences, Lydia Gabriela Speyer, PhD, said in an editorial published with the study. Dr. Speyer is affiliated with the department of psychology at the University of Cambridge (England).
“Given the novel finding that insufficient sleep is associated with changes in brain structure and connectivity that are long-lasting, early intervention is crucial because such neural changes are probably not reversible and might consequently affect adolescents’ development into adulthood,” Dr. Speyer wrote.
Delaying school start times could be one way to help kids get more sleep. The American Academy of Pediatrics and the American Academy of Sleep Medicine recommend that middle schools and high schools start no earlier than 8:30 a.m. to better align with students’ circadian rhythm, Dr. Speyer noted.
As it is in the United States, most schools start closer to 8 a.m. In California, though, a law that went into effect on July 1 prohibits high schools from starting before 8:30 a.m. Other states are weighing similar legislation.
The research was supported by the National Institutes of Health. Dr. Wang and his coauthors and Dr. Speyer had no conflict of interest disclosures. Dr. Williamson is a sleep expert for the Pediatric Sleep Council (www.babysleep.com), which provides free information about early childhood sleep, but she does not receive compensation for this role.