Research and Discoveries

Night Owl Chronotypes in a Morning Lark World

  • Published21 Dec 2020
  • Author Alexis Wnuk
  • Source BrainFacts/SfN
Bird sitting on branch
An internal clock dictates whether you’re a morning lark or a night owl. While many genes contribute to this biological rhythm, or chronotype, our environment seems to be the biggest determinant of when and how much we sleep.
Marette Bennett/

Access to electricity has offered new opportunities for things at night. In doing so, it’s pushed our sleep timing back and expanded the range of individuals’ sleep preferences. But, work and school schedules haven’t really changed.

Lighting Up the Night

For two weeks in 2016, residents of two towns in Mozambique donned activity trackers. The wrist-worn devices recorded their movement, sleep, and light exposure as they went about their day. Those living in the rural village of Tengua spent long days in the fields where they grew maize and tended livestock. Others lived and worked about 20 miles away in the small market town Milange, the only locale in the larger district with access to electricity.

The residents were taking part in a study led by an international team of researchers examining how access to electricity affects sleep. Light sets our internal clocks, signaling that it’s time for activity. It stood to reason that more light at night could disrupt sleep patterns, says study author Kristen Knutson.

Indeed, Milange residents went to bed and woke up almost an hour later than people in Tengua. But they didn’t lose any sleep or sleep poorly as a result — their sleep timing simply shifted later. Access to electricity may change when we go to bed but “it doesn’t necessarily have to lead to poor sleep quality,” the Northwestern Feinberg School of Medicine’s Knutson says.

Surprisingly, it was the Tengua villagers who slept worse, likely because they slept on mats on the floor and slept with more family members to a room. To Knutson, the study highlights the challenges of examining how a single factor — access to electricity — affects sleep. “In understanding sleep patterns, you really need to consider the socio-cultural and environmental factors and context in which people are sleeping,” Knutson says.

The study offers a microcosmic glimpse of the way our environment affects our sleep — for better or worse — and how sleep patterns have changed over the course of human history.

Our circadian rhythms — the roughly 24-hour cycles of sleep and wake — are baked into our genes. Even our preferred bedtime is rooted in our DNA. In the last three decades, scientists have identified a slew of genes linked to chronotype, our individual internal clock that governs when we sleep and when we feel most alert. The Mozambique study, and others like it, show how malleable and adaptable our circadian rhythms are. Access to electricity has pushed our sleep timing back and expanded the range of individuals’ sleep preferences. But, in a world built for morning larks, night owls and even middle-of-the-road chronotypes can experience disastrous health consequences.

Nurture Over Nature

Nearly all living things — from humans to fruit flies to bacteria — run on a roughly 24-hour internal clock. Typos in the genetic code can speed up or slow down the clock; other tweaks shift it earlier or later.

In the 1970s and 80s, CalTech scientists Seymour Benzer and Ronald Konopka identified a genetic mutation in fruit flies that compressed the circadian clock. Another lengthened it. And, yet another abolished the circadian rhythm altogether. Jerry Feldman, then at SUNY Albany and now at UC Santa Cruz, found similar mutations in Neurospora crassa, a type of red bread mold.

Similar genetic variants have been found in people. In 2001, for instance, scientists at the University of Utah identified a genetic mutation responsible for a disabling circadian disruption called familial advanced sleep phase syndrome. People with this condition are extreme morning larks — they wake up around 4:30 a.m. and feel sleepy by 7 or 8 in the evening.

Fruit fly
In the 1970s and 80s, CalTech scientists Seymour Benzer and Ronald Konopka identified genetic mutations in fruit flies that compressed and lengthened the circadian clock, and another that stopped circadian rhythms altogether.
Sebastian Janicki/

While circadian disruptions like these — resulting from a single, misprinted gene — offer scientists a unique opportunity to study the genetic basis of sleep timing, they are rare. For most people, the genetic picture is more muddled. Large-scale genetic studies have uncovered a host of genes linked to normal variations in sleep timing, but each likely exerts only a small effect on its own.

Rather, it seems our environment and behavior interact with our genes to drive when and how much we sleep. They also account for the tremendous variability we now see in chronotypes. “It’s nothing that we genetically or epigenetically changed in us,” says chronobiologist Till Roenneberg of the Ludwig Maximilian University of Munich.

At one point in human history, the sleep timing of morning larks and night owls was probably only a few hours apart, he says. Electricity and other modern amenities have pulled the two apart. Now, a bell-curve distribution with nearly 12 hours separates the extreme morning larks from the extreme night owls but most people fall squarely in the middle.

Chronotype and frequency graph
Chronobiologist Til Roenneberg devised a new method for assessing chronotypes. Rather than sorting people into discrete “morning” and “evening” types, it calculates the midpoint of their sleep on days off, correcting for any sleep debt accumulated during the week.
Fischer et al., 2017

Sleep researcher Kenneth Wright’s work best illustrates the powerful influence of the lighting environment. In 2013, the University of Colorado at Boulder neuroscientist took eight people backcountry camping in the Rocky Mountains. After one week without any electric light or electronic devices, participants’ sleep timing synchronized to the natural light-dark cycle and the differences between night owls and morning larks almost disappeared.

“When you have access to electricity, people can drift one way or the other based on their preference,” Knutson says. “We don’t go to bed the second we feel sleepy. We override that a lot more than other animals do, and so we can shape when we sleep.”

Living Against the Clock

Artificial light and electric devices have shifted our internal clocks, pushing bedtimes back and widening the spectrum of chronotypes. But our social clock, as Roenneberg calls it, has failed to keep pace: Work and social schedules favor early risers, a one-size-fits-all approach that hurts everyone else.

Evening chronotypes experience higher rates of obesity, diabetes, metabolic syndrome, and depression. They’re also more likely to smoke, use drugs, and eat poor diets. And, in a 2018 study of more than 430,000 people, Knutson and her University of Surrey collaborator, Malcolm von Schantz, found evening chronotypes had an increased risk of death during the study period compared to morning types.

Knutson doesn’t take this to mean that being an evening chronotype is inherently dangerous, however. Rather, it’s that they’re fighting against their natural biological rhythms to meet society’s expectations. To put it simply, “it’s being a night owl in a morning lark world.”

As we spend less time outside during the day and more time in front of screens at night, more of us fall into the night owl camp. In a 2012 study, Roenneberg analyzed sleep surveys collected over nine years and found the average chronotype shifted about 15 minutes later, a small but statistically significant difference. “We’ve created an environment that forces the [body] clock to be late,” Roenneberg says.

Chronotype and age graph
Chronotypes change over the lifetime, reaching peak lateness around age 19 before shifting earlier. They also differ by sex: in epidemiological studies, Roenneberg has found women (red shading) have earlier chronotypes than men (blue shading) until midlife, after which men shift earlier.
Fischer et al., 2017

As we cut our sleep short during the work week, we accumulate sleep debt. Sleeping in on the weekends erases the debt but reverts our circadian rhythms to their natural set point. Come Monday, we have to reset — a phenomenon Roenneberg calls “social jetlag.”

Maintaining a consistent sleep schedule by waking up earlier on the weekends fails to solve the problem. “It’s not sleeping in on the weekend but getting up too early during the week that makes us sick,” he says.

Roenneberg proposes adjusting work and school schedules to better match individuals’ internal clocks. His own research indicates chronotypes change over the lifetime and reach peak lateness during the teenage years. A 7:30 a.m. school start time is essentially the middle of the night for teenagers.

It’s also becoming clear that our internal clocks govern more than our sleep. Daily oscillations in metabolism and hormone secretion also determine optimal times for eating and exercising.

“The science is coming out to suggest, hey, timing matters, too,” says Knutson. “In the idea of a healthy lifestyle — with diet, exercise, and sleep — it isn’t just how much or what, but when as well.”

Content Provided By


1. How has access to electricity changed sleep patterns?
2. What role do genes play in circadian rhythms and chronotypes?
3. How does social jetlag hurt our health?

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