Some people get up before their alarm, fresh and efficient. Others need an hour and two coffees before they can string three words together. For a long time, this was seen as a matter of willpower, or even character: early risers were disciplined, night owls a bit lazy. Biology tells a very different story.
A clock in every living thing
In 1729, the French astronomer Jean-Jacques Dortous de Mairan ran a simple experiment. He shut a mimosa plant in a dark cupboard and noticed that it kept opening its leaves during the day and folding them at night, without seeing any light. The rhythm came from within.
More than two centuries later, in 1962, the caver Michel Siffre spent two months alone at the bottom of a chasm in the southern French Alps, with no watch and no daylight. He ate and slept whenever he felt like it. When he came out, he thought it was August 20: it was September 14. Without cues, his days had stretched and he had "lost" several weeks.
These observations have a name: the circadian rhythm, from the Latin circa diem, "about a day". In humans, the conductor is a small group of about 20,000 neurons in the hypothalamus, the suprachiasmatic nucleus. Left to itself, this clock runs on average slightly slower than 24 hours. It is light, picked up by the retina, that resets it every day.
In 2017, the Nobel Prize in Physiology or Medicine went to Jeffrey Hall, Michael Rosbash and Michael Young, who discovered, in fruit flies, the genes and proteins that make this clock tick inside every cell.
Larks and owls
Everyone has a clock, but it is not set to the same time for everyone. This is what researchers call the chronotype: each person's natural place between "larks", who are morning people, and "owls", who are evening people.
Two questionnaires are the standard for measuring it. The one by James Horne and Olov Östberg, published in 1976, asks among other things what time you would get up if you were entirely free to plan your day. The one by German chronobiologist Till Roenneberg, developed in the early 2000s, looks mainly at the midpoint of sleep on free days, when no alarm goes off.
The result: chronotypes form a continuum. Most people sit somewhere in the middle, and the true extremes, those who naturally fall asleep at 9 pm or at 3 am, are a minority.
A matter of genes… and age
Chronotype is partly inherited. In 2019, a study of nearly 700,000 people identified 351 regions of the genome associated with being a morning person. Each one weighs very little, but together they explain part of the differences between individuals. The natural length of the clock plays a role too: in evening people, it tends to run a little slower.
Age, meanwhile, shakes things up for everyone. Young children are often true early risers. In adolescence, the clock shifts markedly towards the evening: it is not just the fault of screens, it is biological. This shift peaks around age 20, so clearly that Till Roenneberg suggested treating it as a biological marker of the end of adolescence. After that, people gradually become earlier again as they age.
Social jetlag
The problem is that society is set up for larks. Schools, offices and public transport start early. Evening people go to bed late because their clock wants them to, get up early because their schedule demands it, and catch up at the weekend.
Till Roenneberg gave this phenomenon a name: social jetlag. Every Monday, part of the population goes through the equivalent of a transatlantic flight. Several studies link it to shorter sleep, higher consumption of coffee or tobacco, and difficulty concentrating.
Teenagers are the clearest case. In 2014, the American Academy of Pediatrics recommended that middle and high schools start no earlier than 8:30 am. When Seattle high schools pushed their start time back from 7:50 to 8:45 am in 2016, students slept about half an hour more on average, and their grades in the classes studied improved.
Can you become a morning person?
A little, but not completely. The clock is set mainly by light:
- morning light moves the clock earlier: going out early, opening the curtains wide, having your coffee by a window helps you fall asleep earlier at night;
- evening light moves it later: screens, bright lighting and late nights push back the moment you feel sleepy;
- regularity matters: getting up at roughly the same time every day, weekends included, limits the Monday jetlag.
These habits can shift your rhythm by an hour or two. They will not turn a committed owl into a lark. And there is no need: being an evening person is not a flaw. Research even shows that people do demanding tasks better when they fall at their peak time.
What about your mornings?
Our morning energy quiz does not replace researchers' questionnaires, but it draws on them: it looks at how you wake up, your rituals and your energy to place you between the early bird, the slow riser, the morning chameleon and the night owl.
In short
- Every human being has an internal clock, reset every day by light.
- Chronotype, morning or evening, depends partly on genes and changes with age, with an "evening" peak around 20.
- Because society is set up for early risers, evening people live with permanent social jetlag.
- You can move your clock a little earlier with morning light and regular hours, without changing your nature. It is better to organize your days with your rhythm than against it.