Morning or evening: what the chronotype does and does not say

There is a personal typology that contemporary science accepts, measures and publishes without reservation. It deserves our attention, because it settles a question of principle.

What has been established

In 1976, two researchers, Horne and Östberg, published a questionnaire in the International Journal of Chronobiology designed to place a person on a morningness–eveningness axis.

It has become a standard. It is used in sleep centres around the world, its French version is distributed through specialist networks, and it has appeared in research protocols for half a century.

What it establishes is simple and considerable: time-of-day preference is not a habit, nor a matter of discipline. It is a characteristic of the person, largely innate, stable over time. You can work against it. You cannot change it.

What this settles

An objection of principle one often hears: that a personal typology would, by its very nature, be a pre-scientific way of thinking.

This is not the case. Chronobiology publishes one, measures it, and treats it as a fact. The work of Franz Halberg, who coined the term “circadian” in 1959, opened an entire field in which the rhythms specific to each organism — including those of the skin — are documented.

The question, then, is not whether people differ in stable ways. They do. The question is how we read that difference.

What the chronotype does not say

It tells you at what hour you function. It does not tell you what you spend, what you hold on to, how you recover, or what your skin produces or loses.

It is one axis, a single one, and it claims nothing more.

The point where it stops

On method.

The chronotype is read through a questionnaire — that is, through what the subject declares. This is perfectly acceptable for what it measures, and it carries two known limitations: you answer according to your current life, and you answer according to the idea you have of yourself.

The same questionnaire filled in three years apart, in a different professional situation, does not always yield the same result. What is supposed to be stable is read by an instrument that, itself, is not.

What the tradition proposed

A broader reading — not one axis but several registers, and not only schedules: what you spend, what you retain, where you give way, how you restore yourself.

And a key of another order. A reading that begins from a datum no one declares and nothing can vary, one that depends neither on the mood of the day nor on the circumstances of the moment.

That is the difference between measuring something stable with a variable instrument, and reading it directly.