The riddle of the Sphinx
In the Greek myth, the Sphinx stopped travelers with a riddle: what walks on four legs in the morning, two legs at midday, and three in the evening? Oedipus got it. A human being, crawling as a baby, walking as an adult, leaning on a cane at the end. William Dement opens his chapter on sleep across a lifetime with that riddle, because sleep pulls the same trick. The way we sleep at the start of life barely resembles the way we sleep in the middle, and the end looks like a third creature entirely.
In part one I wrote about sleep debt, the loan shark in my head, and the clock that fights it all day. This is part two, and like every part two since The Empire Strikes Back, things get more complicated and somebody loses a hand. In this case the hand belongs to my twenties.
Do babies dream in the womb?
Sleep starts before we do. Late in pregnancy, a fetus is asleep something like 16 to 20 hours a day, and Dement notes it's more active asleep than awake, which explains why so many pregnant women get kicked at 3 a.m. The stage doing most of the work is , the stage most closely tied to dreaming.
Newborns spend about half their sleep in REM, roughly eight hours a day. Babies born early spend even more. Adults spend about a quarter of the night there, and by old age it falls to somewhere around 15 to 20 percent. Whatever REM is for, we front-load it.
Do babies actually dream? Nobody can ask them. Dement got the closest thing to an answer from his own daughter, who wasn't yet two. He heard her talking in her sleep one morning ("Pick me, pick me"), woke her, and she told him, "Oh, Daddy, I was a flower." I have now put four flowers into this series so far, and I'm officially blaming her.
The clock starts early too. As a young intern on an obstetrics rotation in 1957, Dement noticed the delivery rooms were quiet in the afternoon and frantic in the middle of the night. Later he learned why: the mother's body clock acts like a gatekeeper on labor, holding it back during the day and waving it through at night, which for most of history meant giving birth while predators slept.
When do babies start sleeping through the night?
A newborn sleeps 16 to 18 hours a day, broken into six or seven short stretches spread around the clock. Their sleep cycles run about 60 minutes instead of an adult's 90. To exhausted parents it all looks completely random.
It isn't random. In the 1930s, Nathaniel Kleitman (yes, the same man who spent a month in a Kentucky cave in part one) asked parents to log exactly when their baby was asleep and awake, day after day, for months. When you stack those daily records on top of each other, a pattern jumps out. The baby's most wakeful stretch arrived a little later every day, sliding roughly an hour later each week, the same longer-than-24-hour drift the German bunker volunteers showed as adults.
Then, around the seventeenth week, the drift stopped. The baby's waking window snapped into place at the same time every day. The clock had found the sun. Dement's own first child was what people then called a "colicky baby," and he admits he was extremely envious of parents whose babies slept through the night early.
From there, sleep consolidates fast. By the end of the first year a baby sleeps around 14 to 15 hours, mostly at night. By 18 months most toddlers are down to one nap. Between ages two and five, the nap fades out, and kids settle at around ten hours of sleep a night until puberty.
Every baby is different. For anything about your own child's sleep, your pediatrician is the right call.
Is it ADHD, or is my kid just tired?
In 1975, Dement lost his biggest research grant and had to lay off almost his entire staff. A private foundation saved him, with one condition: the research had to be on children. That accident gave us the Stanford Summer Sleep Camp, where in 1976 Dement and Mary Carskadon started following kids aged 10 to 12 for a decade.
What they found in the preteens was almost comic. On the daytime nap test from part one, where you get 20 minutes to fall asleep, these kids basically never did. Not once, all day. At night they slept about ten hours, deeply, and then woke up like puppies. Dement calls this age "life's pinnacle of perfection" when it comes to sleep and wakefulness.
The flip side is what happens when kids don't get it. Sleepy adults slow down. Sleepy kids often speed up. Dement writes that children often get louder, wilder, and more rambunctious as they get tired, then crash so hard they're nearly impossible to wake. Then he writes a sentence that stopped me: with more and more children being diagnosed with ADHD, he thought there was "a good possibility that some of these children are really just sleepy."
That line is from 1999, and it needs context. ADHD is a real, well-studied condition, and the science of diagnosing it has come a long way since then. But his core point has held up: too little sleep, or a sleep disorder like sleep apnea, can produce restlessness, poor focus, and impulsive behavior that looks a lot like ADHD, and it can make true ADHD worse. Today, sleep problems are commonly part of what doctors look into when a child is struggling to pay attention. Reading that passage as an adult with ADHD, I mostly wondered what my own nine-year-old nap test would have shown.
If you're worried about a child's attention or sleep, start with their pediatrician. This is one writer's reading of the research, not a diagnosis.
Why do teenagers stay up so late, and how much sleep do they need?
Before the camp, everyone assumed teenagers needed less sleep than kids. The camp kept the same children coming back every summer, so Dement and Carskadon got to watch them grow up. At 13, 14 and 15, given the chance, the kids still slept the same ten hours as before. Yet their daytime test scores showed them getting sleepier. The only explanation was that teenagers need at least as much sleep as younger children, and probably more.
Carskadon's later work at Brown University found the second half of the story. During puberty, the body clock itself shifts later. Teenagers typically don't start feeling sleepy until an hour or more after the adults around them. Their late nights are less rebellion and more biology, which is a sentence every parent of a teenager should have tattooed somewhere visible. Dement also cites a survey in which only five percent of American high school students said their parents set a bedtime. So the clock says midnight, nobody says otherwise, and first period starts at 7:20.
That 7:20 start was real, in Edina, Minnesota, until the district pushed it to 8:30. A University of Minnesota study led by Kyla Wahlstrom followed what happened. Students were more awake and engaged in class, and behavioral problems went down. California went further and passed a law in 2019 requiring most public high schools to start no earlier than 8:30, which took effect in 2022.
The cost of getting this wrong goes past grades. Dement cites survey data showing 53 percent of people aged 18 to 29 reported daytime sleepiness, compared with 33 percent of adults over 30, and 72 percent of 18-to-24-year-olds said they had driven drowsy in the past year. Today the American Academy of Sleep Medicine recommends 8 to 10 hours a night for teens aged 13 to 18. Most don't get close.
Why is sleep so hard with ADHD?
Dement only brushed past ADHD, and only in children. But reading his chapters on the mask and the clock, I had the uncomfortable feeling he was describing my adult brain's entire operating system.
An ADHD brain hunts for stimulation. It's under-stimulated by the boring thing and wildly over-stimulated by the interesting thing, and the interesting thing almost always shows up at 11:40 p.m. A brain built to chase stimulation is, by design, a brain built to hide its own sleep debt. I had been wearing the mask for so long I forgot it was a mask.
The research points at the clock as well as the habits. A 2010 study in Biological Psychiatry found that adults with ADHD and chronic trouble falling asleep had a delayed , meaning the body's chemical "lights out" signal arrives late. Later work from the same Dutch group estimates that as many as 78 percent of adults with ADHD have a delayed , with the signal running about an hour and a half behind. Ninety minutes, every night, between when the world tells you to sleep and when your body lets you. Think back to the teenagers in the last section. The late clock that arrives with puberty is supposed to drift back toward normal in your twenties. For a lot of adults with ADHD, these studies suggest, it stays parked late.
Dement saw this pattern decades before anyone linked it to ADHD. In 1975 a Stanford graduate student came to his clinic because he lay awake every night and only fell asleep as everyone else was starting their day. Most doctors at the time would have called it anxiety. Dement called it "phase lag syndrome," a biological clock running behind the world. Today the diagnosis is , and it's the most common sleep problem in adults with ADHD. Dement's fix was wonderfully strange: since the student's clock wanted to run late, they had him go to bed a few hours later every day, walking his bedtime all the way around the clock until it landed at 10 p.m. It worked.
In another survey of 120 adults with ADHD, 78 percent said they struggled to get to bed at a reasonable time and often stayed up until between 1 and 3 a.m. Nearly 70 percent had trouble falling asleep, about the same share struggled to get up in the morning, and most said it had been that way their whole lives. I could have filled out that survey at nine years old.
That theft has a name now: . You've given the day to everyone else, so you take the night back for yourself, one more episode, one more scroll, one more idea. With ADHD, the night is when the noise finally quiets and the brain finally locks in, and every one of those golden hours is borrowed against tomorrow at loan shark rates. The levers Dement described still work. A 2021 Dutch clinical trial found that morning bright light combined with carefully timed melatonin moved the delayed clock in adults with ADHD earlier by up to two hours.
Researchers are still working out which way the arrow points: whether ADHD drives sleep problems, whether sleep problems worsen ADHD symptoms, or both. This is my experience plus what the studies say, not a diagnosis or a treatment plan. Melatonin timing in those trials was set by clinicians. If any of this sounds like you, a doctor can help sort out what's going on.
When does middle age start? (Stanford students have an answer)
Dement once asked a lecture hall of about 500 Stanford undergraduates to raise their hands if they thought people aged 50 and over were middle-aged. Every hand went up. Forty? No change. Thirty? More than half the hands stayed up. Only at 25 did the room finally let go, with about ten percent still holding firm. I am 32. By the standards of that room, I'm a few years into middle age and should be shopping for reading glasses and a sensible sedan.
The cruel part is that the students weren't entirely wrong. Dement points out that athletes start to decline in their early twenties, fertility measurably begins to drop in the early thirties, and muscle and bone start their slow retreat not long after. His read is that nature, having gotten us to reproductive age, stops investing in the upkeep.
Middle age is also the least-studied stretch of sleep, for a reason Dement finds ironic: people in their thirties, forties and fifties are too busy earning money and raising families to spend a week in a sleep lab. The data that does exist, from researchers Dan Kripke and Sonia Ancoli-Israel in San Diego, found middle-aged adults averaging about seven hours a night, less than older teens and people in their twenties. Dement also cites a survey in which more than 80 percent of Americans said you can't be a success at work and still get enough sleep. In the entertainment business, I'd bet that number rounds up to a hundred.
Can you catch up on sleep on the weekend?
The book is 27 years old, and on this question it needs an update. Dement's ledger holds up: the debt is real and it accumulates. Newer research complicates the refund policy.
In 2019, Kenneth Wright, the same Colorado researcher behind the camping study in part one, led a study where one group slept five hours a night on weekdays and then slept as much as they wanted on the weekend. The weekend catch-up didn't reset their metabolism. By some measures they ended up worse than the people who never got a weekend reprieve at all. Dement himself described an NIH experiment, led by psychiatrist Thomas Wehr, that hints at how deep the hole goes. Volunteers who spent fourteen hours a night in darkness for weeks slept enormous amounts at first and took about three weeks to settle into a steady eight hours and fifteen minutes. By Dement's reading, they had walked in carrying roughly thirty hours of debt without noticing it. They felt perfectly normal when they arrived.
Weekend lie-ins have a second cost, and Dement spells it out. Because the clock naturally runs long, sleeping until noon on Saturday and Sunday pushes it even later, so Monday morning your body is living a few time zones west of your alarm. He calls the result Sunday night insomnia and Monday morning blues. Researchers today call it , and you don't need a passport to get it.
So you can't sleep twelve hours on Saturday and call it even. The smarter move is the boring one: stop taking out new loans, and keep the same wake time all seven days.
Why do older people wake up so early?
In 1979, Dement and Carskadon brought a group of healthy adults aged 65 to 88 to the summer camp. These were carefully screened, normal older sleepers. The results surprised them anyway. More than 40 percent had some form of , and a majority had extraordinarily frequent , tiny awakenings of three seconds or less that they never remembered, anywhere from 200 to 1,000 times a night.
The clock changes too. Teenagers drift into owls, and older adults drift into larks, falling asleep earlier and waking before dawn. Charles Czeisler at Harvard, who you met in part one, found one reason: in older sleepers, the clock's nightly dip isn't as deep, so its wake-up signal breaks through around 4 or 5 a.m. Dement lived this himself. A committed night owl as a young doctor, by 70 he found his most productive hours were roughly 3 or 4 a.m. to 9 or 10, with a full day's work done by lunch.
He also pushed back on the old idea that older people simply need less sleep. Their ability to sleep declines faster than their need for it. Dement closed the chapter hoping for medicine that could restore the deep sleep of our preteen years. He lived to 91.
Paying it down at 32
Something happens in your early thirties. The body that used to forgive everything starts sending invoices. For me it came all at once: a new office job with a real start time, a body that recovered slower than it used to, and the slow realization that I was managing my ADHD everywhere except the place it was doing the most damage.
I started taking my health seriously across the board, and sleep turned out to be the account underneath all the others. Here's what I'm actually doing:
Hollywood will never greenlight the biopic of a man who started going to bed at a reasonable hour. But the Sphinx's riddle has a fourth line Dement never wrote down: somewhere between two legs and three, you get to decide how you'll sleep for the rest of the walk.
The loan shark still works my block. I've stopped borrowing from him.
Questions people search
How much sleep do teenagers need?
The American Academy of Sleep Medicine recommends 8 to 10 hours a night for teens aged 13 to 18. Stanford research found teenagers need at least as much sleep as younger children, even though they tend to get less.
Why do teenagers stay up so late?
During puberty the body clock shifts later, so most teens don't feel sleepy until an hour or more after adults do. Early school start times collide with that shift, which is why many districts, and the state of California, have moved high school start times later.
Do babies dream?
Newborns spend about half their sleep in REM, the stage tied to dreaming in adults, and fetuses spend much of their sleep there too. Whether that counts as dreaming in the way adults experience it is impossible to confirm, since babies can't tell us.
When do babies get on a sleep schedule?
In Nathaniel Kleitman's classic record from the 1930s, a baby's sleep drifted later each day for months before locking onto a 24-hour rhythm around the seventeenth week. Every baby is different, so your pediatrician is the best guide.
Can sleep deprivation look like ADHD?
Yes. Too little sleep, or a sleep disorder such as sleep apnea, can cause restlessness, poor focus and impulsive behavior, especially in children, and can worsen true ADHD. That's why sleep is often part of an attention evaluation.
Does ADHD cause insomnia?
Sleep problems are very common with ADHD. Studies of adults with ADHD find many have a delayed body clock that makes it hard to fall asleep at a normal time. Researchers are still untangling cause and effect.
Can you catch up on lost sleep?
Partly. Recovery sleep helps after a few bad nights, but research suggests weekend catch-up doesn't fully undo a week of short sleep, and sleeping in late shifts your clock. Consistency works better than binge sleeping.
Do older people need less sleep?
Not necessarily. Dement's research suggested the ability to sleep declines with age faster than the need for it. Older adults wake more often during the night and their body clocks shift earlier, which is why many wake before dawn.
Sources
- William C. Dement and Christopher Vaughan, The Promise of Sleep (Delacorte Press, 1999), chapters 3, 4 and 5.
- Van Veen et al., "Delayed Circadian Rhythm in Adults with ADHD and Chronic Sleep-Onset Insomnia," Biological Psychiatry 67(11), 2010.
- Bijlenga, Kooij et al., "Adult ADHD and the circadian rhythm," European Psychiatry (survey of 120 adults with ADHD).
- van Andel et al., "Effects of chronotherapy on circadian rhythm and ADHD symptoms in adults with ADHD and delayed sleep phase syndrome," Chronobiology International, 2021.
- Depner, Wright et al., "Ad libitum weekend recovery sleep fails to prevent metabolic dysregulation," Current Biology, 2019.
- Wahlstrom, K., University of Minnesota research on later high school start times, Edina, Minnesota.
- California Senate Bill 328 (2019), school start times.
- American Academy of Sleep Medicine, sleep duration recommendations for teens and adults.
I'm a guy who reads a lot, not a doctor. Nothing here is medical advice. If sleep is wrecking your days, or your kid's, talk to a physician or a sleep specialist.