The Science Of How Sleep Changes Your Brain, From Infancy To Old Age
Why getting the best sleep is even more important than you thought.
The role of sleep changes with every stage of life, from infancy to old age. The latest neuroscience is discovering how crucial sleep is to an infant’s growing brain, while the latest epidemiology is discovering how irregular sleep doubles the risk of death as we grow older. To mark National Sleep Week, Thrive Global spoke with some of the top researchers in sleep science to give you a map of how sleep changes through your lifespan.
What scientists are discovering about sleep through the ages is fascinating, like how sleep helps the brain lay down the equivalent of fiber-optic cable before you’re even born to the way “social jet lag” affects the lives of primary schoolers to why you have trouble staying asleep as you get older.
With that said, let’s dive in.
Infancy: When sleep helps build your brain.
The need for and power of sleep starts showing up before you even properly enter the world. Beginning in the third trimester of pregnancy, a fetus starts exhibiting what looks like rapid eye movement (REM) sleep, which, in adults, is when dreaming occurs and memories are stored. For fetuses, neurons are growing rapidly — it’s like “an internet service provider laying down high-speed fiber optic cable within the brain,” says Matthew Walker, PhD, the principal investigator at the University of California, Berkeley’s Sleep and Neuroimaging Laboratory. Even before a baby is born, it already has circadian rhythms, or the “body clock” that determines your wakefulness and sleepiness throughout the day.
Once we’re out in the world, sleep becomes our primary activity. On average, a newborn infant sleeps 16–17 hours a day and a six-month-old sleeps 13–14 hours a day. In that first year of life, a baby spends more than half of its time sleeping. As Thrive Global founder and CEO Arianna Huffington notes in her book The Sleep Revolution, infants spend about half of their sleep in REM, a rate that falls to about 20 percent after their first birthday and stays stable into adulthood.
Research suggests that, among other things, sleep deprivation in an infant undermines the brain’s “plasticity,” or the ability of the organ to rewire itself, allowing it to better adapt to whatever life is throwing at it (which is, of course, quite a lot, what with this being a whole new world and all.) In a trend that will hold for the rest of our lives, sleep supports the formation of memories and learning new things early in life.
Childhood: It’s all about consistency.
Once kids reach grade school age, the links between sleep and behavior become startlingly clear. The National Sleep Foundation recommends 9 to 11 hours of sleep per night for primary schoolers. But it’s not just the amount of sleep kids get that matters — regularity is crucial, too. Digging into a large-scale British national survey of some 10,000 kids between ages 3 and 11, University College London epidemiologist Yvonne Kelly and her colleagues found that inconsistent bedtimes wreak all sorts of havoc on a growing child.
The results were striking: As Kelly and her colleagues reported in a 2013 paper, variable bedtimes were linked to lower scores on math, reading and spatial awareness tests. Another paper of hers published that same year found that kids with irregular bedtimes were evaluated as having worse social behavior by their mothers and teachers. Then a 2016 follow-up reported that children with irregular bedtimes were more likely to be overweight and have lower self-esteem and satisfaction with their bodies.
The key to understanding all this, Kelly told Thrive Global, is circadian rhythms. “If I traveled from London to New York, when I get to you I’m likely to be slightly ragged,” she says, as that jet lag is not only going to harm her cognitive abilities, but also her appetite and emotions (red-eyes don’t make for charming company). “That’s in adults,” she continues, “but if I bring one of my children with me and I want them to perform on a math test having just jumped across time zones, they will struggle even more than I will.” The body is an instrument, and a child’s is especially prone to getting out of tune.
That’s what happens when kids go to bed at 8 p.m. one night, 10 p.m. the next and 7 p.m. another — researchers call this a “social jet lag effect.” Without ever getting on a plane, a child’s bodily systems get shuffled through time zones and their circadian rhythms and hormonal systems take a hit as a result. Not coincidentally, other research has found that poor sleep in childhood puts kids at risk for emotional and behavioral problems in adolescence and beyond.
Teenhood: When society sets you up to be sleep-deprived.
As anyone who’s been through it will tell you, adolescence is weird, and that weirdness extends to sleep. Just like children, teens need consistency in their sleep — brain imaging research suggests that teens with variable sleeping patterns have less density in their white matter, which carries signals between neurons and otherwise acts as connective tissue in the brain. That density should increase as teens grow, meaning that irregular sleep may get in the way of the brain’s development in learning and attention.
The National Sleep Foundation recommends 8 to 10 hours of sleep a night for teens, which might not be that hard to get if it weren’t for what researchers call “sleep phase delay.” When they hit puberty, teens get tired up to an hour or more later than they did as children. Indeed, the teen tendency toward late sleep is so strong that scientists propose that moving back to an earlier bedtime, which we naturally do around age 20, is a sign that adolescence is ending.
Naturally late bedtimes mean teens also tend to sleep in later in the morning. The researchers I talked to were careful to emphasize that teens aren’t lazy for sleeping later — that’s what their physiology is begging them to do. This is also why early school days are such a disaster. In one formative study, Brown University sleep scientist Mary Carskadon and her colleagues recruited 40 high schoolers who usually started their day at 7:20 a.m. or 8:25 a.m., depending on their grade. After monitoring them for a couple weeks, the researchers brought the teens into a lab on a Saturday and tested them during what would have been their second period on a typical school day. “Half of them looked like they had narcolepsy,” she says. “We put them to bed and they were asleep in under a minute — their brains wanted them and likely needed them to be asleep in seconds.”
The stakes for sleepy teens aren’t just an inability to learn: Unintentional injuries are the leading cause of death for adolescents, with about two-thirds of these injuries involving car crashes. A 2016 report from the Centers for Disease Control and Prevention found that underslept teens were more likely to text while driving, drink and drive, ride in a car with a drinking driver, not wear a seatbelt and infrequently wear a bicycle helmet. Developmental neuroscience is confirming that teens are already predisposed to risk, and lack of sleep doesn’t help.
As Arianna has argued, this is part of the reason why we need to move to later school start times, which, studies show, have led to improved well-being and academic achievement in high schoolers. Society stigmatizes sleep in teenagers, says Matthew Walker, and that creates “an incendiary situation for the adolescent brain.” Teens need lots of sleep, they naturally go to bed later and they’re still expected to show up to class early. And if they sleep in on the weekend, they’re told they’re “wasting the day.” It’s a “societal tragedy,” Walker says. Teen sleep deprivation has turned into a national public health issue, and thankfully, the movement for later school start times is gaining traction.
The college and post-grad years: Sleep links with achievement.
Psychologists are beginning to refer to the period after adolescence, up to around ages 25–29, as “emerging adulthood.” Given their proximity to the academics who study sleep, much of the research on this age group has been on college students. In line with the research on younger people, studies link quality sleep with better health and grades. Among college students, poor sleep has been linked to greater anxiety and depressive symptoms, more binge eating and lower GPA scores. As with teens, college students would also likely fare better with later start times. (The National Sleep Foundation recommends 7 to 9 hours of sleep per night for this age group.)
While the sleep research on emerging adults who are out of college is still early, what’s been discovered is frankly shocking, namely the 2016 finding that people in this age group with sleep problems are more likely to be verbally or physically aggressive with their romantic partners than their better-rested peers — another sign that sleep is essential for emotional stability. Other research suggests that sleep deprivation makes it harder for people to recognize and express their emotions while also increasing emotional reactivity. Brain imaging indicates that in the sleep-deprived brain, the danger-sensitive amygdala and the behavior-controlling prefrontal cortex don’t communicate well, making you more likely to overreact to perceived threats.
Adulthood: Responsibility, exhaustion and hormones are messing with your rest.
Pregnancy can cause shifts in your sleep schedule, and, of course, becoming a mother or father puts you at risk you for sleep deprivation.(Babies don’t seem to care about the link between slumber and emotional stability.) Indeed, exhaustion is a key risk factor for postpartum depression, which an estimated one in nine American women develop after giving birth. Other research finds that women wake up more readily to the sound of a crying baby, suggesting a sensitivity baked deeply into evolutionary roles; relatedly, new moms get less sleep than new dads, producing what researchers are calling “baby-induced fatigue” at work. Gender roles still largely dictate that it’s first the woman’s job to take care of the baby in the middle of the night, though the rise of male lead parents is helping to change that norm.
Amy Wolfson, PhD and author of The Woman’s Book of Sleep, says that biological sex and social gender roles each play a role in sleep as people grow into adulthood, though it’s a relatively new field of inquiry. “The understanding of hormones and sleep didn’t occur until the end of the 20th century,” she says. “Ironically, one of the reasons women were left out of traditional research studies is that researchers were worried that someone might menstruate or get pregnant, and yet those are the very issues that affect sleep.”
In women, menopause can cause sleep problems, including disorders like insomnia, especially if they experience hot flashes or other temperature changes that wake them up at night. Men are more likely to develop certain sleep disorders, like sleep apnea, where breathing starts and stops irregularly — 24 percent of middle-aged men have it, compared with nine percent of women.
Lots of research shows that getting way less (or even way more) sleep in middle age increases the likelihood of dying prematurely. A 2007 British study of almost 10,000 civil service workers between the ages of 35 and 55 found, after a seventeen year follow-up, that shifts up or down in sleep time were linked to being twice as likely to die during the study. (Those who went from seven to five or fewer hours of sleep per night doubled their risk of death from cardiovascular issues, for example.) Similarly, a 2014 American study of 130,000 people also found that, over a thirteen year span, getting more or less sleep increased participants’ risk of death. And a 2010 analysis of 16 studies — with a total of 1.3 million participants — found again that short and long sleepers both had a greater risk of death, with the authors concluding that between six and eight hours a night was the sweet spot.
Getting more than that could signal that you have a yet-to-be detected disease, the researchers say, and getting five or less puts you at higher risk of death overall. (The National Sleep Foundation recommends seven to nine hours a night for adults age 26 to 64, and seven to eight hours for those over 65.) The question of whether short or long sleep is a cause of or marker for illness remains an important research question.
Old Age: Sleep quality goes down, sleepiness goes up.
Simply put, the brain gets worse at sleeping as you get older. Matthew Walker, the University of California neuroscientist, says that your quantity of sleep declines by the time you hit 65, as electrical signals that help keep your brain asleep — called sleep spindles — decrease by up to half and nighttime bathroom trips become more frequent.
On an equally unnerving note, non-REM sleep, which is critical for your immune system, memory and other cognitive processes, “gets demolished” as you get older, Walker says — its activity declines by 40 to 50 percent. (Older men suffer much greater losses of this kind of sleep, also known as “slow wave,” than women.) Because of all these sleep irregularities, older adults also tend to be sleepier through the day — a quarter of them report being so tired that it interferes with their daytime activities. REM sleep doesn’t start dropping off until your 80s, or in conjunction with a degenerative disease like Alzheimer’s, Walker and his colleagues note in a 2017 review of the literature. All of this adds up to the unavoidable fact that your sleep gets worse as you get older.
While there aren’t really any clinical interventions to help mitigate that loss in non-REM sleep, the steps for maintaining healthy sleep are much the same as you’d tell a high schooler: avoid alcohol, stop drinking so much coffee in the afternoon and perhaps most important of all, be consistent with your bedtimes. Because if there’s one thing the human body hates — whether you’re 7, 27 or 77 — it’s giving yourself jet lag.
In a physiological sense, sleep is where your body finds balance for its many functions, from the emotional to the cognitive, all the way down to the immune system. While it happens out of sight, sleep is full of life-giving action. Just like you want to eat right, you want to sleep right.