Growth, development and sleep — The World, with the Evidence

Painting: at night, in a desert under the moon, a woman lies asleep beside a mandolin and a jug while a lion sniffs at her. — The Sleeping Gypsy (La Bohémienne endormie), painting by Henri Rousseau (1897). Henri Rousseau, Public domain, source

Having trouble falling asleep early and trouble getting up, in adolescence, is not laziness.

It has been measured, it has been dated, and it is not a matter of opinion. This page explains how we know.

What happens

As adolescence approaches, the body’s internal clock shifts.¹

Falling asleep and waking up naturally move later. It is not that adolescents decide to go to bed late: it is that the biological signal that triggers sleep arrives later than before.

How we know it is biological

Everything could be explained otherwise: adolescents go to bed late because they have homework, screens, friends, and because they like staying up. That would be a purely behavioural explanation, and it seems plausible.

So researchers set up situations in which the two explanations do not predict the same thing.

First test: Adolescents are put on regular schedules, with enough time to sleep. If the shift came only from habits, it should disappear. It persists.
Second test: They are observed in the laboratory, under controlled conditions where social influence is limited. Same thing: if it were social, it should fade away. The shift remains.
Third test: In 1993, a team questions more than four hundred pupils of the same age group about their morning or evening preference, and looks for what it is linked to. The researchers compared it with pubertal development, but also with two social factors: the type of school — and so whether or not the pupils mix with older pupils — and birth order.²

Result: a correlation with pubertal development. No link with the two social factors.³

Three ways of trying to prove the biological explanation wrong. None of them succeeds.

This is how a claim becomes solid: not because it is repeated, but because those who defend it have themselves designed the tests capable of refuting it.

What this study does not say

Since this page claims to teach how to read a result, it owes it to itself to read this one all the way through.

The link is clear in girls. In boys, the same trend appears — but it does not reach the threshold of significance.⁴

That does not mean it is absent: it means that, in this particular sample, it cannot be asserted.

The caution of the authors in their own conclusion also deserves to be noted. They do not write “we have demonstrated”. They write that these data support involvement of a biological factor.⁵

That is why the two other tests matter. One partial result plus another partial result that supports it by a different route: that is how a conclusion is built. Rarely all at once, almost never through a single study.

The part we control, and the part we do not

We must be careful not to draw the opposite conclusion. The shift is partly kept up by whatever pushes bedtime back: homework, screens, social life. So it is neither all biological nor all behavioural. It is both.⁶

Telling yourself “it’s biological, there’s nothing I can do about it” gives you an alibi, and it is false: part of it depends on what you do in the evening.
Being told “it’s just slackness” gives you guilt, and that is false too: part of it does not depend on you and will not disappear through sheer willpower.

There is a part we do not control and a part we do. It is by telling one from the other that we can act. And if sleep difficulties are severe or lasting, it is better to talk to a health professional about them.

A public debate, and what should be said about it

This finding has led, in several countries, to discussion of the time at which lessons start.

This site does not take sides:

What is: Does the internal clock shift in adolescence? This question is settled by measurement, and it is settled.
What should be done: Should lessons start later? This question has no single scientific answer. It involves transport, parents’ work, evening activities, the organisation of schools and the budget.

A scientific measurement never says what should be done. It says what is happening. What is then done with it is a political and social decision, and decisions are open to debate.

On the function of sleep and its safeguards

Why do we sleep? The question is still being studied. In 2013, a study carried out on mice concluded that sleep speeds up the cleaning of the brain; in 2024, another, also carried out on mice, measured the opposite. The debate concerns this mechanism: that sleep is important is not disputed. Measured facts must also be distinguished from interpretations: sleep is not a magic formula to be optimised at all costs with gadgets.⁷

Screens: they are part of what pushes bedtime back, like homework and social life.⁸

On growth

This page gives no curve, no figure, no reference age, no range. No description of what would be “ahead” or “behind”.

Rates of growth vary enormously from one person to another. That is not the exception, it is the rule.

Two people of the same age can be at very different stages of development, while both being perfectly fine. An average is not a target to be reached: it is a statistical calculation on a population. Nobody is an average.

When you have questions about your own body, the right thing to do is to talk to a health professional who can examine you and listen.

What we owe the reader

This is the last page of this encyclopaedia, and we want to conclude with our commitments:

Not to frighten. Fear does not make people think.
Not to lie. Neither by painting too dark a picture, nor by reassuring without proof.
Not to hand out missions. The reader is not responsible for repairing what others have done.
Never to ask to be taken at our word.

Every page states where its information comes from, every figure carries its date, and every doubt is made explicit. The reader does not need to trust us: the reader needs to be able to check.

Note 1

Moreover, both home-based and laboratory studies of adolescents show that delayed circadian phase correlates with secondary-sex development [7, 10, 11].

Hagenauer, M. H., Perryman, J. I., Lee, T. M. and Carskadon, M. A., “Adolescent Changes in the Homeostatic and Circadian Regulation of Sleep”, Developmental Neuroscience, vol. 31, no. 4, 2009, p. 276-284

See the full note ↓

Note 2

Adolescents continue to show a delayed circadian (or internal clock) phase as indicated by daily endocrine rhythms […] even after several weeks of regulated schedules that allow for sufficient sleep

This delay is maintained under controlled laboratory conditions in which there is limited possibility for social influence

183 sixth-grade boys and 275 sixth-grade girls completed questionnaires for morningness/eveningness (M/E) and pubertal status