Dr. Avi Sadeh of Tel Aviv University is one of the authorities in the field. A couple of years ago, Sadeh sent 77 fourth-graders and sixth-graders home with randomly drawn instructions to either go to bed earlier or stay up later for three nights. Each child was given an actigraph (a wristwatchlike device that’s equivalent to a seismograph for sleep activity), which enabled Sadeh’s team to learn that the first group managed to get 30 minutes more sleep per night. The latter got 31 minutes less sleep.
After the third night’s sleep, a researcher went to the school in the morning to test the children’s neurobiological functioning. The test they used is highly predictive of both achievement-test scores and how teachers will rate a child’s ability to maintain attention in class.
Sadeh knew that his experiment was a big risk. “The last situation I wanted to be in was reporting to my grantors, ‘Well, I deprived the subjects of only an hour, and there was no measurable effect at all, sorry—but can I have some more money for my other experiments?’” he says.
Sadeh needn’t have worried. The effect was indeed measurable—and sizable. The performance gap caused by an hour’s difference in sleep was bigger than the normal gap between a fourth-grader and a sixth-grader. Which is another way of saying that a slightly sleepy sixth-grader will perform in class like a mere fourth-grader. “A loss of one hour of sleep is equivalent to [the loss of] two years of cognitive maturation and development,” Sadeh explains.
Sadeh’s findings are consistent with other researchers’ work, all of which points to the large academic consequences of small sleep differences. Dr. Monique LeBourgeois of Brown University studies how sleep affects pre-kindergartners. Virtually all young children are allowed to stay up late on Fridays and Saturdays. Yet she’s discovered that the sleep-shift factor alone is correlated with performance on a standardized school-readiness test. Every hour of weekend shift costs students seven points on the test. Dr. Paul Suratt of the University of Virginia studied the impact of sleep problems on vocabulary-test scores of elementary-school students. He also found a seven-point reduction in scores. Seven points, Suratt notes, is significant: “Sleep disorders can impair children’s I.Q.’s as much as lead exposure.”
Every study done shows a similar connection between sleep and school grades—from a study of second- and third-graders in Chappaqua to a study of eighth-graders in Chicago. The correlations really spike in high school, because that’s when there’s a steep drop-off in kids’ sleep. Dr. Kyla Wahlstrom of the University of Minnesota surveyed more than 7,000 high schoolers in Minnesota about their sleep habits and grades. Teens who received A’s averaged about fifteen more minutes sleep than the B students, who in turn averaged eleven more minutes than the C’s, and the C’s had ten more minutes than the D’s. Wahlstrom’s data was an almost perfect replication of results from an earlier study of more than 3,000 Rhode Island high schoolers by Brown’s Mary Carskadon. Certainly, these are averages, but the consistency of the two studies stands out. Every fifteen minutes counts.
With the benefit of functional MRI scans, researchers are now starting to understand exactly how sleep loss impairs a child’s brain. Tired children can’t remember what they just learned, for instance, because neurons lose their plasticity, becoming incapable of forming the synaptic connections necessary to encode a memory.
A different mechanism causes children to be inattentive in class. Sleep loss debilitates our body’s ability to extract glucose from the bloodstream. Without this stream of basic energy, one part of the brain suffers more than the rest: the prefrontal cortex, which is responsible for what’s called “executive function.” Among these executive functions are the orchestration of thoughts to fulfill a goal, the prediction of outcomes, and perceiving consequences of actions. So tired people have difficulty with impulse control, and their abstract goals like studying take a back seat to more entertaining diversions. A tired brain perseverates—it gets stuck on a wrong answer and can’t come up with a more creative solution, repeatedly returning to the same answer it already knows is erroneous.
Convinced by the mountain of studies, a handful of school districts around the nation are starting school later in the morning. The best known of these is in Edina, Minnesota, an affluent suburb of Minneapolis, where the high school start time was changed from 7:25 a.m. to 8:30. The results were startling. In the year preceding the time change, math and verbal SAT scores for the top 10 percent of Edina’s students averaged 1288. A year later, the top 10 percent averaged 1500, an increase that couldn’t be attributed to any other variable. “Truly flabbergasting,” said Brian O’Reilly, the College Board’s executive director for SAT Program Relations, on hearing the results.
Another trailblazing school district is Lexington, Kentucky’s, which also moved its start time an hour later. After the time change, teenage car accidents in Lexington were down 16 percent. The rest of the state showed a 9 percent rise.