Chronotype: are you a lark, a dove or an owl?

Claire teaches at a secondary school, and by half past five in the morning she has done the shopping and finished her first coffee. At nine, when her second lesson starts, she is the best version of herself. The colleague in the next classroom barely speaks before half past eight, yet at nine in the evening he marks a whole stack of tests in a time Claire could not match after eight if her career depended on it. The difference between them has a name, and the name is chronotype.

Both sleep around seven hours and both are perfectly healthy. What separates them is not discipline, and not who takes mornings more seriously. It is the timing of their internal clock, and that timing belongs among the most stable things you can know about your own body.

An internal clock with no reset button

Deep in the brain, in the hypothalamus, sits a cluster of cells about the size of a grain of rice. It keeps a cycle close to twenty-four hours, though in most people slightly longer. Shut someone in a bunker with no windows and no watch, and they would fall asleep a little later every day, until after a few weeks they were living in a completely different time from everyone outside.

Light is the reason that does not happen to you. Morning light hitting the retina pulls the clock earlier; evening light pushes it later. The whole daily cycle is called the circadian rhythm, from the Latin "circa diem", meaning "about a day". Your chronotype describes one specific thing: where your personal cycle sits relative to the clock on the wall.

Sleep is only the most visible part of it. Body temperature moves across roughly one degree during the day and bottoms out a couple of hours before you would naturally wake. Shortly after waking, cortisol spikes and gets the body up and running, while melatonin, the hormone your body makes on its own, starts rising about two hours before your natural bedtime.

The master clock in the hypothalamus is not the only one ticking, either. The liver, the gut and the muscles run daily rhythms of their own, and those follow not just light but also when you eat. A late dinner sends the body a signal that says the day is still on, and fires up digestion exactly when the rest of the system is winding down. The same meal at six in the evening and at midnight does not behave the same way.

Attention, reaction speed, appetite and mood all ride the same wave. That is why an eight o'clock meeting strikes one person as an entirely sensible idea while another leaves it feeling they were never really there. Which hours give you the best shot at serious mental work is the subject of a separate piece on your most productive time of day.

Lark, dove, owl: three bands on one scale

Chronotypes are usually described in three bands. They are not boxes with hard edges, more like stretches of a continuous scale running from the earliest people to the latest. Most of the population sits somewhere in the middle, and the pronounced ends on either side are a minority.

The dove is not an average in the sense of nothing special. It is the position from which manoeuvring is easiest: an early meeting and an evening shift are about equally mildly unpleasant. There is one price for that, and it is fuzziness. The dove has no sharply defined peak of its own, so it has to be protected deliberately, or somebody else's calendar will take it.

The spread of people along this scale is bell-shaped. Pronounced larks and pronounced owls together make up a minority, and the overwhelming bulk of the population fits into a broad middle, leaning one way or the other. Most of us are therefore neither bird in full. We are more of an owl, or a mild lark.

The tricky part is how often people file themselves wrongly. Anyone who goes to bed at two calls themselves an owl, but a large share of them are simply staring at a phone until two, and their own clock would have put them under at eleven. The opposite mistake is just as common. Someone who has been getting up at six for fifteen years because they have to gets used to saying they are an early bird, even though every morning starts in fog. A chronotype is not what you do, it is what your body would do if left alone.

The extreme bands have it the other way round. They know exactly when they are at their best, and they run straight into a world calibrated for the middle. What an ordinary working week looks like for a pronounced owl, and what can be done about it, is covered in the piece on being a night owl; the other end of the scale gets its own article on early birds.

In a couple, the difference gets sharper still. The owl is barely warming up while the lark is already yawning, which leaves a narrow window for a shared evening. None of that has to hurt a relationship, as long as both know that at ten at night they are not two equally tuned people. There is more on that in the article on how a night owl and an early bird live alongside each other.

Where a chronotype comes from

Genes carry the largest share. Twin studies compare how closely identical twins match on morningness and eveningness against fraternal pairs, and their heritability estimates land somewhere around half. The rest goes to age and to how much light reaches you, and at what hour.

Behind that heritability sits a particular family of genes governing how fast certain proteins are built up and broken down inside a cell. That loop is effectively the ticking, and small differences in its speed shift the whole cycle earlier or later. Which is why a chronotype tends to run visibly through a family. If you are up at five and so are your parents, upbringing is probably not the explanation.

Age is the factor that surprises almost everyone. Small children are larks with barely an exception, as any parent of a preschooler can confirm. In puberty, though, the rhythm starts drifting to later hours, and it keeps drifting until somewhere around the age of twenty.

Till Roenneberg and his team at the university in Munich collected sleep data from tens of thousands of people and, in 2004, read something unexpected out of it. The chronotype is at its latest around the age of twenty, in women slightly earlier than in men, and after that the trend reverses. For the rest of life, a person drifts slowly back toward the morning. Roenneberg proposed treating that turning point as a biological marker that adolescence has ended.

Plenty follows from that in practice. A seventeen-year-old who cannot fall asleep before midnight and is useless at seven is not necessarily lazy. He happens to be sitting near the top of a shift that will fade on its own within a few years. And a grandmother padding around the flat at six is not morally better off, she is simply further along the same road.

The third layer is light and how we handle it. Outdoors, even under thick cloud, several thousand lux reach the eyes; in an ordinary office it is a few hundred. The body therefore gets a weak signal in the morning and a strong one at night, because screens and ceiling lights stay on late. Populations as a whole run later than their genetics would suggest, and people who work outdoors tend on average to be earlier types than those who spend the day inside.

The season has a say too. In winter, when mornings are dark and you get outside barely at lunchtime, the rhythm slides later for many people, only to return in summer. That is not a disorder, just a response to how much light the eyes take in over a day. Pronounced owls feel the winter shift most, because they start from a later position and December pushes their morning further away still.

Does a chronotype change, and can it be retrained?

The first answer is easy. Yes, it changes reliably, though not by decision. It changes with age. The chronotype you had at twenty is not the one you will have at forty-five, and the gap can easily be an hour or more.

The second question is trickier, and the honest answer is "halfway". You can move your rhythm a fair distance. Bright light right after waking and dim evenings can drag your clock by roughly an hour, in some people a bit more. Flipping an owl into a lark, on the other hand, does not happen. Two weeks of holiday or illness are enough for the rhythm to slide back where it was.

When you do want to move it, one route works, and it is thoroughly boring. You shift in steps of fifteen to twenty minutes a day, not hours, and the body needs to see the new schedule for at least a fortnight running. Mornings are helped by stepping outside, or at minimum sitting by a window; evenings by cutting bright light about two hours before your target bedtime. What decides the outcome more than anything else, though, is a fixed wake-up time seven days a week, because one weekend lie-in can wipe out a whole week of work.

That distinction between moving and rewriting is worth holding on to, because a lot of good intentions break on it. Set an alarm two hours earlier than your body will even let you fall asleep and you do not gain two hours. You gain two hours of sleep loss, and they add up night after night.

The gap between when you sleep on working days and when you sleep with nobody making demands has a name: social jetlag. Roenneberg coined it, and the term is meant literally. Every Monday morning, the body of a pronounced owl goes through something much like a flight across several time zones, minus the ticket and minus anyone finding it strange.

The second axis: steady or flexible rhythm

Morningness and eveningness is only one coordinate. The second asks something entirely different: how much does a change of schedule throw you? This is where it is decided whether shift work will be bearable for you or purgatory.

Someone with a steady rhythm needs regularity and knows it immediately. One late night bleeds into the following two days, and an eastbound flight flattens them for a week. Inside a settled routine, though, they run like clockwork and know exactly what to expect of themselves at any hour.

A flexible rhythm means the opposite. A shift is absorbed quickly, and a trip halfway around the world costs one scrambled day rather than a week of aftermath. The price is less predictability. Such a person can rely less on Tuesday at ten in the morning going the way Monday at ten did.

Which group you belong to shows up in a single question: what does one sleepless night do to you? Some people are tired the next day and fine the day after. Others are still assembling themselves by the end of the week and hunting for words on Thursday. For planning a life, the answer to that often matters more than whether you are an owl.

The practical consequence is fairly concrete. A steady rhythm pays to protect at the edges: hold the same wake-up time and turn down the late event rather than run at half throttle for three days afterwards. A flexible rhythm can afford rotating schedules and work in another time zone, but should not scramble the routine for no reason, because tolerance has a ceiling as well.

The two axes are independent of each other. There are owls with a steady rhythm whom any change knocks flat, and larks who fly to America and function normally the next day. Two numbers therefore say far more than one label: the first describes when, the second how easily that can be moved.

How to work out your own chronotype

The best data is at home and free. All it takes is watching what the body does at a moment when nobody is pushing it.

A holiday is the finest laboratory for this. Discount the first two or three days, when the body is catching up on what you have owed it all year. From the fourth day onward, something usually settles that is genuinely yours. Note when you fell asleep and when you woke without an alarm, then work out the midpoint between the two. For a pronounced lark it lands somewhere near two in the morning; for a pronounced owl it can easily be five.

Now be honest: if nobody expected anything of you tomorrow morning, no meeting, no children to get to school, what time would you get up? Not what time you would like to get up, and not what time you get up now. What time your body would wake by itself.

Small details from an ordinary week help as well. How long after waking you want food, or whether nothing but coffee gets into you for the first hour. What time in the evening the moment arrives when your brain shuts off regardless of what is left on the list. And when you last did something difficult genuinely well, rather than merely handing it in.

Two things distort the picture for almost everybody, though. The first is sleep debt: chronically short sleepers drop off anywhere and any time, so their own rhythm hides underneath the exhaustion. The second is caffeine, because a cup at three in the afternoon can push falling asleep back by an hour, after which the person swears they are an owl when they are nothing of the sort. Before measuring anything, give yourself a week with no coffee after noon and an honest amount of sleep.

If after all that you are still unsure where you sit on both axes, a starting bearing comes from our chronotype test. It asks about ordinary situations, takes a few minutes, and its result is worth treating as a launching point for your own observation rather than a verdict.

Two weeks that will tell you more than any questionnaire

Try one exercise for a fortnight. Pick a single activity you do almost daily and that costs you something, say the piece of writing you keep avoiding, or an uncomfortable phone call. Every time, jot down the hour and a mark from one to five for how well it went.

After two weeks a curve emerges from those numbers, with one clear hump and one hole. For most people, both sit somewhere other than expected. Almost everyone guesses their peak from when they have the most work on, not from when their head works best. That one hour the graph hands you is worth never lending to anybody.