Night Owl in a Lark's World: Why Mornings Are Rigged, and What It Costs

Every office, school and gym in the world opens on a lark's schedule, and about half the people walking into them are running on a different clock. The word for that clock is chronotype, and the word for the tiredness it produces in an owl on a nine-to-five is not laziness. It is jet lag, taken every Monday, without leaving town.

The science here is unusually clear and unusually ignored. Chronotype is a trait. It is about half inherited. It moves through life on a curve that has been mapped in tens of thousands of people. And almost none of that has reached the timetable.

What a chronotype is

Chronotype is the time the body prefers to sleep and wake when nothing is forcing it. The clean way to measure it, which Till Roenneberg's group in Munich developed, is to ignore the working week entirely and take the midpoint of sleep on free days. An adult who sleeps from midnight to eight on a Saturday has a mid-sleep of four. One who sleeps from two to ten has a mid-sleep of six, and is an owl.

The distribution is a bell curve with a long evening tail. In the largest American study, 53,689 diaries from the American Time Use Survey, a quarter of adults had a weekend mid-sleep earlier than 2:24am, half fell between 2:24 and 4:15, and the remaining quarter were later than that: the owls. The entry on chronotype has the mechanics; this piece is about the collision.

The curve nobody tells teenagers about

The single most useful fact in the chronotype literature is that lateness peaks at about nineteen and declines for the rest of life. The Time Use Survey data show it plainly.

A twenty-year-old's body wants to be mid-sleep at nearly five in the morning. A seventy-year-old's wants it before three. That is a two-hour swing across a lifetime, and more than half of it happens between fifteen and twenty-five. The teenager who cannot get up is not being difficult; the retiree awake at five is not being virtuous. Both are where the curve puts them.

Seattle tested what happens when a timetable bends toward the curve. In 2017 the district moved high school start times from 7:50 to 8:45, and a team led by Gideon Dunster measured the students before and after with wrist monitors. Sleep went up by a median of 34 minutes a night. Grades rose 4.5 percent. At the more disadvantaged of the two schools, attendance and punctuality improved too. The students had not changed. The clock on the wall had.

Social jetlag, the cost with a number

Roenneberg and Marc Wittmann coined the term in 2006 for the gap between the sleep the body wants and the sleep the week allows. It is measured the same way as chronotype: take the mid-sleep on free days, take the mid-sleep on work days, and subtract. For a mild owl on an early schedule the gap is an hour. For a hard one it is two or three, every week, for a working life.

By Roenneberg's 2012 estimate, two-thirds of people in Western societies carry at least an hour of it. The same study, across tens of thousands of participants, found social jetlag associated with higher body weight independent of how long people slept, and the wider literature adds worse mood, more caffeine and more nicotine. The mechanism is not mysterious. Chronic misalignment means eating, working and trying to sleep at hours the body has scheduled for something else.

Two things about that finding are worth holding onto. The associations are population level, not a diagnosis of any one owl. And they attach to the mismatch, not the trait. An owl who works evenings, or negotiates a late start, has an owl's clock and none of the jetlag.

Three clocks, one household

Where the collision gets personal is the shared bed. Most couples are not matched on chronotype, and the mismatch looks like something it is not.

A 2018 study of 91 couples by Jocz, Stolarski and Jankowski found that mismatch in chronotype predicted lower relationship satisfaction for the women in the sample, and that a gap in preferred time for intimacy predicted lower sexual satisfaction for both partners. A larger dyadic study in Chronobiology International in 2022 found matched couples reporting better sleep and greater sexual satisfaction than mismatched ones. The effects are moderate and the samples are not huge. What they describe is recognisable to anyone who has lived it: the only hours the owl feels like a person are the hours the lark is already unconscious.

The failure mode in a mismatched couple is not the clocks. It is reading the clocks as a verdict. The lark hears rejection at ten at night. The owl hears indifference at seven in the morning. Neither is happening. A scheduling problem is being processed as a relationship problem, and the Conflict Style test tends to show which partner goes around it rather than saying so.

Fighting the clock or negotiating the timetable

Two strategies exist. Only one of them works for long.

The lever that actually moves a chronotype is light. Bright light within minutes of waking pulls the clock earlier; dim evenings stop it drifting later. It shifts things by tens of minutes, sometimes an hour, which for a mild owl is the whole gap and for a hard one is the difference between three hours of jetlag and two. What does not move the clock is willpower, and the research on people who tried is mostly a record of relapse.

Reading your own clock

None of this is treatment. A preference so extreme that ordinary life is impossible, asleep at five every morning for years, is a circadian rhythm disorder, and it belongs with a sleep clinic. What a questionnaire can do is smaller and, for most owls, enough: put a number on the gap.

The Chronotype test asks what would happen if the day were yours and returns where you fall and how hard, because a mild evening lean and a two-hour one need different fixes. Read next to the Sleep Quality test, it says whether the trait is the reason the nights are not working. Read next to a partner's result, it turns the argument about who is being difficult into a picture of two clocks, which is a much easier thing to negotiate with than a person.

The first sign that any of this is working is not becoming a morning person. It is a weekday morning that is merely unpleasant instead of impossible, and a Sunday night without the dread.

Sources

• Fischer, D., Lombardi, D. A., Marucci-Wellman, H., Roenneberg, T. (2017). Chronotypes in the US: influence of age and sex. PLOS ONE, 12(6).
• Roenneberg, T., Allebrandt, K. V., Merrow, M., Vetter, C. (2012). Social jetlag and obesity. Current Biology, 22(10).
• Wittmann, M., Dinich, J., Merrow, M., Roenneberg, T. (2006). Social jetlag: misalignment of biological and social time. Chronobiology International, 23(1-2).
• Dunster, G. P., et al. (2018). Sleepmore in Seattle: later school start times are associated with more sleep and better performance in high school students. Science Advances, 4(12).
• Koskenvuo, M., Hublin, C., Partinen, M., Heikkilä, K., Kaprio, J. (2007). Heritability of diurnal type: a nationwide study of 8753 adult twin pairs. Journal of Sleep Research, 16(2).
• Jocz, P., Stolarski, M., Jankowski, K. S. (2018). Similarity in chronotype and preferred time for sex and its role in relationship quality and sexual satisfaction. Frontiers in Psychology, 9, 443.
• Sleep and sexual satisfaction in couples with matched and mismatched chronotypes: a dyadic cross-sectional study (2022). Chronobiology International, 39(9).
• Roenneberg, T., et al. (2007). Epidemiology of the human circadian clock. Sleep Medicine Reviews, 11(6).
• Wieth, M. B., Zacks, R. T. (2011). Time of day effects on problem solving: when the non-optimal is optimal. Thinking & Reasoning, 17(4).