A different mechanism causes children to be inattentive in class. Sleep loss debilitates the 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, 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.
Both those mechanisms weaken a child’s capacity to learn during the day. But the most exciting science concerns what the brain is up to, when a child is asleep at night. UC Berkeley’s Dr. Matthew Walker explains that during sleep, the brain shifts what it learned that day to more efficient storage regions of the brain. Each stage of sleep plays its own unique role in capturing memories. For example, studying a foreign language requires learning vocabulary, auditory memory of new sounds, and motor skills to correctly enunciate the new word. The vocabulary is synthesized by the hippocampus early in the night during “slow-wave sleep,” a deep slumber without dreams. The motor skills of enunciation are processed during stage 2 non-REM sleep, and the auditory memories are encoded across all stages. Memories that are emotionally laden get processed during REM sleep. The more you learned during the day, the more you need to sleep that night.
To reconsolidate these memories, certain genes appear to upregulate during sleep—they literally turn on, or get activated. One of these genes is essential for synaptic plasticity, the strengthening of neural connections. The brain does synthesize some memories during the day, but they’re enhanced and concretized during the night—new inferences and associations are drawn, leading to insights the next day.
Kids’ sleep is qualitatively different than grownups’ sleep because children spend more than 40% of their asleep time in the slow-wave stage (which is ten times the proportion that older adults spend). This is why a good night’s sleep is so important for long-term learning of vocabulary words, times tables, historical dates, and all other factual minutiae.
Perhaps most fascinating, the emotional context of a memory affects where it gets processed. Negative stimuli get processed by the amygdala; positive or neutral memories gets processed by the hippocampus. Sleep deprivation hits the hippocampus harder than the amygdala. The result is that sleep-deprived people fail to recall pleasant memories, yet recall gloomy memories just fine.
In one experiment by Walker, sleep-deprived college students tried to memorize a list of words. They could remember 81% of the words with a negative connotation, like “cancer.” But they could remember only 31% of the words with a positive or neutral connotation, like “sunshine” or “basket.”
“We have an incendiary situation today,” Walker remarked, “where the intensity of learning that kids are going through is so much greater, yet the amount of sleep they get to process that learning is so much less. If these linear trends continue, the rubber band will soon snap.”
While all kids are impacted by sleep loss, for teenagers, sleep is a special challenge.
Brown’s Mary Carskadon has demonstrated that during puberty, the circadian system—the biological clock—does a “phase shift” that keeps adolescents up later. In prepubescents and grownups, when it gets dark outside, the brain produces melatonin, which makes us sleepy. But adolescent brains don’t release melatonin for another 90 minutes. So even if teenagers are in bed at ten p.m. (which they aren’t), they lie awake, staring at the ceiling.
Awakened at dawn by alarm clocks, teen brains are still releasing melatonin. This pressures them to fall back asleep—either in first period at school or, more dangerously, during the drive to school. Which is one of the reasons young adults are responsible for more than half of the 100,000 “fall asleep” crashes annually.
Persuaded by this research, a few school districts around the nation decided to push back the time school starts in the morning.
The best known of these is Edina, Minnesota, an affluent suburb of Minneapolis, which changed its high school start times from 7:25 to 8:30. The results were startling, and it affected the brightest kids the most. In the year preceding the time change, math/verbal SAT scores for the top 10% of Edina’s 1,600 students averaged 683/605. A year later, the top 10% averaged 739/761. In case you’re too drowsy to do that math, getting another hour of sleep boosted math SAT scores of Edina’s Best and Brightest up 56 points, and their verbal SAT score a whopping 156 points. (“Truly flabbergasting,” gasped a stunned and disbelieving Brian O’Reilly, the College Board’s Executive Director for SAT Program Relations, when he heard the results.) And the students reported higher levels of motivation and lower levels of depression. In short, an hour more of sleep improved students’ quality of life.
That’s particularly remarkable since most kids get less sleep during high school, and their quality of life goes down: University of Kentucky’s Danner has studied how, on a national level, sleep decreases each year during high school. In their first year, 60% of kids got at least eight hours on average. By the second year, that was down to 30%. Right alongside this decline went their moods; dropping below eight hours doubled the rate of clinical-level depression. Over one-eighth of the students reached this classification, which makes one only wonder how many more suffer from melancholy of a lesser degree.
Another trailblazing school district is Lexington, Kentucky, which also moved its start time an hour later. Danner has been studying the before/after equation. The finding that most jumps out from his data is that after the time change, teenage car accidents in Lexington were down 25%, compared to the rest of the state.
While the evidence is compelling, few districts have followed this lead. Conversely, 85% of America’s public high schools start before 8:15 a.m., and 35% start at or before 7:30 a.m.
Obstacles against later start times are numerous. Having high schools start earlier often allows buses to first deliver the older students, then do a second run with the younger children. So starting later could mean doubling the size of the bus fleet. Teachers prefer driving to school before other commuters clog the roads. Coaches worry their student-athletes will miss games because they’re still in their class at kickoff time. Many simply aren’t persuaded by the science. When Westchester schools declined an initiative to start high schools later, then-superintendent Dr. Karen McCarthy opined, “There’s still something that doesn’t click for me.”
Dr. Mark Mahowald has heard all those arguments. As Director of the Minnesota Regional Sleep Disorders Center, he’s been at the center of many school start time debates. But of all the arguments he’s heard, no one’s argument is that children learn more at 7:15 a.m. than at 8:30. Instead, he forcefully reasons, schools are scheduled for adult convenience: there’s no educational reason we start schools as early as we do. “If schools are for education, then we should promote learning instead of interfere with it,” he challenges.
“We thought the evidence was staggering,” Carole Young-Kleinfeld recalled.
Kleinfeld is a mother in Wilton, Connecticut, thirty miles up I-95 from New York City. Wilton, too, had saved money by running buses in two shifts, starting the high school at 7:35. Then a few years ago, Kleinfeld was at a meeting for the local League of Women Voters. Then-state senator Kevin Sullivan spoke about Carskadon and others’ research, and how starting high school at a more reasonable hour was the answer.
Kleinfeld had a sullen teenager of her own, and when she went to local high schools to register kids to vote, she regularly saw students sleeping in the halls during class. So the idea hit home. She and others formed a committee to learn about the issue. Eventually, they convinced the district to move the high school’s start time to 8:20.
For Kleinfeld, the change “was a godsend.”
Her son Zach had once been a perfectly happy kid, but when he hit high school he became the prototypical disengaged, unenthralled-by-everything teen. He was so negative, so withdrawn that “I really thought we’d lost him,” Kleinfeld sighed. “We’d lost that sense of connection.”
After the high school start time shifted, Kleinfeld couldn’t believe it. “We got our kid back.” Zack would bound downstairs in the morning with a smile, wanting to share a funny story he’d read in The Onion. His SAT scores went up, too.
Several scholars have noted that many hallmark traits of modern adolescence—moodiness, impulsiveness, disengagement—are also symptoms of chronic sleep deprivation. Might our culture-wide perception of what it means to be a teenager be unwittingly skewed by the fact they don’t get enough sleep?
University of Pittsburgh’s Dr. Ronald Dahl agrees, observing: “Is it adding one percent or sixty percent, we don’t know. But clearly a lack of sleep makes it much worse.”