A Child's Sleep Differs From an Adult's

A baby sleeping soundly in your arms opens their eyes the moment you set them down on the mattress. A few years later, that same child cannot get up even after ten wake-up calls each morning.

What parents struggle with looks like the exact opposite in each case. One is hard because the child will not sleep; the other is hard because the child will not wake. But from the body's side, the two scenes are the same story.

Sleep Keeps Being Rebuilt While a Child Grows

Duration is not the only thing that differs from adult sleep. The doorway into sleep is different, the proportions of each stage within sleep are different, and the rules that set when drowsiness arrives are different. And all three of these keep changing as a child grows.

A Newborn's Sleep Begins in Light Sleep

When an adult falls asleep, they descend first into quiet sleep. A newborn is the opposite.

There is a kind of sleep where the eyes move beneath closed lids, the limbs twitch, and breathing is uneven. This corresponds to REM sleep in adults, and in newborns it is called active sleep. A baby's sleep begins here. It takes around six months for the sequence to shift to entering quiet sleep first, the way an adult does.

The share this kind of sleep takes up out of the whole also differs. Around birth it makes up more than half of total sleep, and it drops below half over the course of the first year.

So the few minutes when a parent goes from holding a baby to setting them down happen to land right at the lightest part of sleep. It is not because there is something wrong with the baby's back — sleep at that stage is simply built this way. I covered this scene separately in why babies wake when put down.

The baby does not wake because you do not know how to lay them down. If you see it only as a matter of technique, you miss the fact that sleep itself is structured this way.

In Puberty, the Hour Drowsiness Arrives Shifts Later

A few years later, the same child is wide awake at night and cannot get up in the morning.

Melatonin, which used to rise in the evening, now begins rising later at this stage. It is not that the child has decided to stay up late — the hour at which drowsiness arrives has itself shifted later.

Habit alone is not enough to explain this shift. This kind of delay around puberty is not unique to humans — it is observed across a number of animal species as well. That means the same thing happens in places with no phones and no late-night schedules.

The more a body matures, the more slowly drowsiness builds up during waking hours. This drowsiness that accumulates with time spent awake is called sleep pressure. For the same number of waking hours, a child at this stage feels less sleepy. That is why it becomes easier for them to stay up later than they could when younger.

The clock shifts later while drowsiness builds slowly. This is why waking a child up in the morning becomes especially hard from this stage onward. A teenager who cannot get up in the morning is a matter that has nothing to do with diligence.

The Same Thing Arrives at Different Ages

A baby who sleeps too lightly and a teenager who sleeps too late look, to a parent, like opposites. But both are simply sleep built according to the rules of that age, and the problem appears where those rules clash with the household's daily schedule.

Between these two scenes, things keep changing continuously. A baby's sleep, once scattered throughout the day, consolidates into the night over the course of months, the share of light sleep shrinks, and then in puberty, the clock shifts later.

So ‘they never used to be like this’ is an accurate observation. The child has not changed — sleep at that age has changed.

Light Nudges the Clock a Little

The shifting of the clock at that age cannot be eliminated altogether. But how far it clashes with the household's schedule can be reduced.

What acts most strongly here is light. Getting the child to see outdoor light, even briefly, right after waking pulls the clock earlier, while a bright screen late at night pushes it later.

If a child sleeps until noon on Saturday, the body's clock takes that late hour as its new reference point. That is why, even after a weekend of plenty of sleep, Monday morning ends up being the hardest. It helps to keep any weekend sleep-in within two hours of the weekday wake time, even if they do sleep late.

When to Check Elsewhere First

If a child snores loudly and sleeps with their mouth open, appears to stop breathing during sleep, or is continually sleepy or listless during the day despite getting enough sleep hours — the problem may not be when they fall asleep, but their breathing while asleep.

Children often do not look sleepy the way adults do. Instead of sleepiness, it can show up as inattention, poor academic performance, or delayed development. The most common cause of sleep apnea in children is enlarged tonsils and adenoids, and it is confirmed with a test that measures the body during sleep (polysomnography).

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The baby who wakes when set down, and the child who cannot get up in the morning, are both simply following the rules of sleep as they are rebuilt at each age. It is only that those rules do not move in step with the household's daily rhythm.

Knowing that much first makes the nights and mornings considerably less noisy. But even when parents come to me with the same complaint, what it points to differs from child to child. I ask when it started, how the night unfolds, and in what order the body switches on after waking in the morning.

Written by Dr. Heo Ji-young (PhD in Korean Medicine Pathology, Kyung Hee University · former Research Professor of Herbology, Kyung Hee University)