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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