Morning person or night owl? What chronotype research says about genes, age and change

Your body has a clock that sets the rhythm of the day, and the most important signal that keeps it in time with the day is the cycle of light and dark. Researchers explain that, because of genetic differences, people’s body clocks may lock on to the same light and dark earlier or later, and that this is what the familiar larks (morning people) and owls (night owls) are; besides genes, they name light exposure and age as causes (Roenneberg et al., 2019). Japan’s Ministry of Health, Labour and Welfare also explains that the light around us is what matters most for setting the body clock. On average the human body clock runs at about 24 hours 10 minutes, a little longer than 24 hours, and people with a longer cycle lean more towards the evening, those with a shorter one more towards the morning (Ministry of Health, Labour and Welfare, 2025).

Morning and evening types are not two separate camps. Within a population, sleep and wake times are spread out close to a bell curve, so that extreme early types wake up when extreme late types fall asleep (Roenneberg et al., 2007), and most people fall somewhere between the two extremes (Wittmann et al., 2006). When the midpoint of weekend sleep was worked out from the diaries of 53,689 people in the American Time Use Survey, it was close to a bell curve overall and within each age group, and half fell between 2:24 and 4:15 a.m. (Fischer et al., 2017). When you sleep and how long you sleep are generally independent (Roenneberg et al., 2007), so being a night owl does not mean you naturally sleep longer.

Research measures morning and evening types in two main ways. The Morningness-Eveningness Questionnaire (MEQ), published in 1976, is a self-report questionnaire evaluated against the temperature rhythms of 48 people; in that study the three types did not differ significantly in how long they slept, but morning types went to bed and got up significantly earlier than evening types, and the authors said it needed further evaluation in a wider population (Horne & Östberg, 1976). The Munich ChronoType Questionnaire (MCTQ) asks when people actually sleep and wake, separately for work days and free days (Roenneberg et al., 2003), and uses as the chronotype the midpoint of sleep on free days, corrected for the sleep debt built up over the working week (MSFsc). The researchers who designed the MCTQ explain that the MEQ asks what time people would prefer, whereas the MCTQ asks about actual behaviour (Roenneberg et al., 2019).

That is why “what share of people are morning types” depends heavily on where the line is drawn. Among 566 middle-aged French employees who did not work shifts, the MEQ’s original cut-offs gave 2.2% evening types, but new cut-offs suited to that age group gave 20.2% (Taillard et al., 2004). In 2,526 New Zealanders aged 30 to 49, the original cut-offs gave 49.8% morning and 5.6% evening types, and the new ones 24.7% morning and 26.4% evening types (Paine et al., 2006). The “type” a questionnaire gives you is best read as a convenient label for a position on a continuum, not as a diagnosis. mind.io has no test that tells you whether you are a morning or an evening type.

Researchers call the gap between getting up to an alarm on work days and sleeping late on free days “social jetlag”. The later the chronotype, the bigger the difference in sleep timing between work days and free days, so sleep debt builds up during the week and is made up on free days (Wittmann et al., 2006). The researchers behind the MCTQ describe many sleep diaries that looked as if people flew several time zones west on Friday evening and back on Monday morning (Roenneberg et al., 2019).

Twin studies suggest that a good part of the differences between people in morningness and eveningness is linked with genes. In 8,753 pairs of adult Finnish twins, genes accounted for about half (49.7%) of the differences between people, and the rest for environmental factors that siblings did not share (Koskenvuo et al., 2007). In 977 pairs of South Korean twins aged 9 to 23, genes explained 45% and the environment 55%, and in all three age groups the effect of being raised in the same family was close to zero (Hur, 2007). It was 44% to 47% in Dutch twin families (Vink et al., 2001) and 0.42 in 1,237 middle-aged American male twins (Toomey et al., 2015). Taken together, the share explained by genes in twin studies is roughly 40% to 50%.

This figure does not mean that “half of one person’s morningness comes from genes”. It describes how far the differences between people in a group go along with genetic differences. Likewise, the finding that the shared family had almost no effect is a statistical result; it does not mean that parents’ routines have nothing to do with a child’s sleep.

Some studies look at genes directly. A study of the genomes of 697,828 people, 449,734 from UK Biobank and 248,098 customers of a personal genetics company, raised the number of genetic loci linked with being a morning person from 24 to 351. But each locus has a small effect: in 85,760 people who wore wrist activity monitors, the 5% carrying the most morningness variants slept on average only about 25 minutes earlier than the 5% carrying the fewest. The genetic differences measured on chips explained 13.7% in UK Biobank, a figure calculated differently from the twin estimates. The participants were of European ancestry, and UK Biobank drew healthier people than average (Jones et al., 2019). So there is no evidence that a gene test can tell you whether you are a morning or an evening type.

It also changes with age. Comparing age groups in the MCTQ data, chronotype gets steadily later from about age 10 to the end of adolescence, around age 20, and then gets earlier for the rest of life (Roenneberg et al., 2019). A 2004 study of about 25,000 people, mostly in German-speaking countries, also reported the latest timing around age 20 (Roenneberg et al., 2004; Fischer et al., 2017), and in the US survey the latest age was 18.4 for women and 19.2 for men. These results compare age groups rather than following the same people. In a 23-year follow-up of 567 Finnish men described by the US survey’s authors, 66% of evening types and 47% of somewhat evening types moved towards the morning, but very few went from clearly evening to clearly morning (Fischer et al., 2017).

Sex differences are very small. In a meta-analysis of 164 questionnaire studies with 186,289 people, men were on average slightly more evening-oriented, but the difference was tiny, a standardised mean difference of about 0.07; an analysis by the studies’ mean age predicted that young women are more morning-oriented than young men and older women less so than older men, suggesting the gap narrows with age (Randler & Engelke, 2019). In the US diary survey, too, men were later before age 40 and women later after it (Fischer et al., 2017). It is not a difference that sorts people into “women are larks, men are owls”.

The body clock moves according to when you get light. Japan’s Ministry of Health, Labour and Welfare explains that light from dawn to around midday shifts the body clock towards the morning (early to bed, early to rise), and light from evening into the middle of the night shifts it towards the evening (staying up late) (Ministry of Health, Labour and Welfare, 2025). In the first MCTQ survey of 500 people, those who reported spending more time outdoors in daylight also fell asleep earlier, though this is an association seen in a questionnaire, not an experiment (Roenneberg et al., 2003). Studies have also found that people without electricity sleep earlier than those with artificial light (Roenneberg et al., 2019).

A few very small experiments show this. Eight people who spent a week of midsummer camping in tents in Colorado, with only sunlight and campfires and no torches or electronic devices, had their body clock markers move about 2 hours earlier and their sleep and wake times about 1.2 hours earlier, and later chronotypes moved the most (Wright et al., 2013). Five people who did the same in winter had their melatonin onset about 2.6 hours earlier than when living under electric light. Nine people who went camping for a weekend saw this onset move 1.4 hours earlier, while five who spent the weekend at home saw it move 0.98 hours later; participants chose their own group, so this was not a randomised experiment (Stothard et al., 2017). This shows that light can move the body clock, not that someone in a city can copy it at home.

One study tested this in everyday life. In a randomised controlled trial with 22 healthy night owls who usually went to bed at 2:30 a.m. and got up at 10:15 a.m. on average, the experimental group spent 3 weeks going to bed and getting up 2 to 3 hours earlier than usual, getting as much outdoor light as possible in the morning, cutting down on light in the evening, keeping the same sleep and wake times on free days, and eating at fixed times. Overall, participants’ sleep and wake times and body clock markers moved about 2 hours earlier with no loss of sleep, and their self-reported depression and stress, as well as their morning reaction times and grip strength, improved (Facer-Childs et al., 2019). According to the abstract, caffeine intake and exercise were also targeted, but it gives no details of these and does not say whether the changes were measured against the control group. We could not check the size of each group or of the changes while writing this guide, and the researchers said it still needs to be shown whether the changes last.

An earlier schedule only lasts if you keep it up. The researchers who designed the MCTQ explain that changing light exposure can bring a night owl’s timing forward, but that it slips back to the late timing as soon as the change stops (Roenneberg et al., 2019). In shift work, the opposite approach has been tried: fitting the schedule to the person. When one factory switched to a timetable that spared extreme night owls the morning shifts and extreme morning people the night shifts, social jetlag fell by 1 hour overall. This was one factory without random assignment, and stress levels did not change (Vetter et al., 2015).

In short, the methods used in research are getting up at a regular time and getting morning daylight, not shifting your wake time much on free days, and cutting down on bright light at night. In the studies above the shift was mostly 1 to 2 hours (about 2.6 hours for melatonin timing in winter camping), and it tends to slip back when people stop. How to make a new wake-up time stick is covered in Forming and breaking habits, and if you find yourself on your phone late at night, see smartphone overuse. If you have bipolar disorder, talk to your doctor before trying light therapy or big changes to your sleep schedule (bipolar disorder).

Studies keep finding that night owls report slightly more depressive symptoms. In a meta-analysis of 36 studies (15,734 people), more evening-oriented people tended to have more severe depressive symptoms, but the link was small (z about −0.20, in the direction of fewer symptoms with higher morningness scores), and the authors said longitudinal studies are needed to work out the direction of cause (Au & Reece, 2017). A pre-registered 2021 meta-analysis pooling 27,996 people also reported a small link (Fisher’s Z −0.24) that varied a lot between studies, with signs of possible publication bias; after adjusting for it the figure was −0.21 (Norbury, 2021).

Why the link exists is still unclear. A study of 1,237 middle-aged American male twins found that morningness-eveningness and depression share part of their genetic basis (genetic correlation −0.21; Toomey et al., 2015). A Mendelian randomisation analysis, which uses genetic variants, found that each genetically predicted shift towards the morning equivalent to a 1-hour earlier sleep midpoint, measured with wrist devices, was associated with about a 23% lower risk of major depressive disorder (odds ratio 0.77, 95% confidence interval 0.63 to 0.94). This compared genetic data from people of European ancestry (170,756 with major depressive disorder and 329,443 controls); nobody actually changed their sleep times. With depression defined from hospital billing codes, the result was not statistically significant, and the authors said randomised clinical trials may be warranted (Daghlas et al., 2021).

Korean data look similar. In an analysis of 46,987 students who answered the 2023 Korea Youth Risk Behavior Survey, students with depressive symptoms had a slightly later midpoint of sleep on free days (corrected for sleep debt) than those without. As a survey at one point in time, it cannot show which came first (Jeong et al., 2025).

Mortality figures need to be read with their comparison. In a study following 433,268 UK Biobank adults aged 38 to 73 for 6.5 years on average, people who answered a single question as “definite evening types” (about 9%) had a 10% higher risk of dying from any cause over that period than “definite morning types” (about 27%) (hazard ratio 1.10, 95% confidence interval 1.02 to 1.18). This was after taking into account age, sex, ethnicity, smoking, body mass index, sleep duration, socioeconomic status and existing illnesses, and 10,534 people died during follow-up. But the two middle groups (moderate morning and moderate evening types) showed no difference, and no group differed from definite morning types in deaths from cardiovascular disease. Split by age, the link was significant only in the 63- to 73-year-olds. As an observational study it cannot show cause, and the participants were healthier than the UK population in general (Knutson & von Schantz, 2018).

Researchers think much of these links may come not from being a night owl as such but from a long-running mismatch between the body clock and society’s schedules for work and school (Wittmann et al., 2006; Knutson & von Schantz, 2018). A large survey found that people with more social jetlag had a higher body mass index, but this too is an association (Roenneberg et al., 2012). If your mood has been low for a long time and your sleep has changed with it, read depression symptoms as well.

There is a link with personality, but it is small. In a meta-analysis of 35 samples with 8,589 people, the personality trait most related to morningness was conscientiousness (r = .29), while openness, extraversion and neuroticism showed only very small links (Tsaousis, 2010). In a meta-analysis of 44 samples with 16,647 people, conscientiousness and morningness scores again had the strongest link (ρ = .37), but the five personality traits together explained only 10.9% to 16.4% of the differences in morningness and eveningness, and eveningness scores had moderate links with extraversion and openness (Lipnevich et al., 2017). In other words, some conscientious people are night owls, and not every morning person is conscientious. The Big Five personality test shows your conscientiousness, but it does not tell you whether you are a morning or an evening type.

What about intelligence? In a meta-analysis of 30 studies (11,160 people), there was no link overall between morningness and intelligence (r = −0.008). A very small positive link between eveningness and intelligence (r = 0.056) also appeared, but it came from samples with a narrower age range and was significant only with some ways of analysing the data (Ujma & Scherrer, 2021).

Night owls tend to do slightly worse at school. In a meta-analysis of 31 studies (27,309 people), more evening-oriented students had slightly lower grades, but the link was small (0.143), and weaker for actual grade records (0.093) than for self-reported grades (0.171) (Tonetti et al., 2015). In a study of 272 German high-school students, more evening-oriented students had lower grades even after taking into account cognitive ability, conscientiousness, achievement motivation and other factors, and the researchers pointed to possible disadvantage for evening-oriented students at school (Preckel et al., 2013). In a survey of 294 first-year Japanese high-school students, a simple comparison found that sleep problems and daytime sleepiness, not chronotype, were linked with a recent drop in grades; in the researchers’ model chronotype could act through sleep problems and sleepiness, but its total effect was not significant (Shimura et al., 2022).

When a test is taken also affects the result. Across studies of attention, memory and other areas, people did best when they worked at the time of day their alertness peaks, a finding known as the “synchrony effect”. Researchers sum up that it is strongest for people with clear morning or evening tendencies and for tasks that need effortful analysis or ignoring distracting information (May et al., 2023). How performance changes over the day depends on chronotype and on whether the time of testing matches a person’s peak (Schmidt et al., 2007). So one early-morning test score says little about a night owl’s ability.

This is why school start times come up so often. The American Academy of Pediatrics named start times before 8:30 a.m. as a key modifiable contributor to teenagers’ lack of sleep and urged middle and high schools to start at times that let students get enough sleep (American Academy of Pediatrics, 2014). A Cochrane review of 11 studies (297,994 students), on the other hand, found that all 6 studies that looked at sleep time reported longer sleep with later school start times, but said the evidence was of very low quality and its effects could not be determined with confidence (Marx et al., 2017). The academy’s statement is a policy position, while the Cochrane review rates the quality of the evidence separately, which is why their verdicts differ. A study of a Korean policy that pushed school start back by 30 to 90 minutes, such as Gyeonggi Province’s 9 a.m. start in 2014, found that the self-reported sleep of 1,133 teenagers followed from grade 7 to grade 12 first rose by 19 minutes, but about a year later the gain had shrunk to 7 minutes and was no longer statistically significant (Jung & Fenelon, 2024). In the US survey, the average midpoint of weekend sleep for 17- and 18-year-olds was 4:30 a.m., which the authors take to mean that high-school students get up and go to school during their biological night (Fischer et al., 2017).

Going to bed late and getting up late is not an illness in itself. But if you have long found it hard to fall asleep and wake up at the times you want, it may be delayed sleep-wake phase disorder (DSWPD), a sleep disorder in which your usual sleep and wake times are delayed, making it hard to fall asleep and wake up at the desired times (Futenma et al., 2023). Japan’s Ministry of Health, Labour and Welfare explains that in typical cases people cannot fall asleep before 3 to 5 a.m. or get up before 9 to 11 a.m. (Ministry of Health, Labour and Welfare, 2025), and that it most often starts between puberty and young adulthood (Ministry of Health, Labour and Welfare, n.d.).

According to a review summarising the criteria of the International Classification of Sleep Disorders (ICSD-3), it is diagnosed when there is a long-standing or recurring complaint of being unable to fall asleep and wake up at the desired or required times; the symptoms have lasted at least 3 months; when free to choose their schedule, people sleep well and long enough for their age but keep the late timing; a sleep diary and, where possible, activity monitoring over at least 7 days (preferably 14) show the late timing; and the problem is not better explained by another sleep disorder, an illness, medication or substance use (Futenma et al., 2023). You cannot diagnose yourself from this guide or from a questionnaire.

How common it is varies a lot between studies. The same review of young people gives an estimate of about 3% without naming the population, while noting that general-population studies reported 0.17% to 1.51%, and that a large Japanese survey put 4.3% of 15- to 30-year-olds at risk (Futenma et al., 2023). In a survey of 10,220 Norwegians aged 16 to 18, 3.3% met the criteria, and more than half of them (53.8% of boys and 57.1% of girls) also met the criteria for insomnia (Sivertsen et al., 2013). A survey of 50,054 Norwegian students (response rate 30.8%) also found 3.3% (Sivertsen et al., 2021). Another review gives a wide range of 1% to 16% in adolescents and young adults, depending on the study (Gradisar & Crowley, 2013). Surveys turn the criteria into questions, so they are not the same as a diagnosis.

Not everyone who sleeps late has this disorder. Japan’s Ministry of Health, Labour and Welfare explains that some university students drift into a similar sleep pattern by living as night owls, but differ in that they can get up when they really must, for an exam or an outing (Ministry of Health, Labour and Welfare, n.d.). A cohort study cited in the review found that, among people whose sleep schedule was delayed, those who met the diagnostic criteria and those who simply slept late without any complaint for more than 3 months were roughly equal in number. And about 40% of people diagnosed with the disorder show no delay in objectively measured body-clock markers, so a late lifestyle is not always down to the body clock alone. The review also considers that the disorder may be an extreme form of eveningness (Futenma et al., 2023).

Diagnosis and treatment are decided with a sleep specialist. Japan’s ministry advises seeing a sleep disorder specialist because treatment needs specialist knowledge (Ministry of Health, Labour and Welfare, 2025). The American Academy of Sleep Medicine guideline endorsed, at a second-tier level of confidence (one step below the strongest), timed melatonin for this disorder and light therapy for it in children and adolescents, and made no recommendation for other treatments or groups because data were insufficient or absent (Auger et al., 2015). This is a separate question from treating insomnia, so rather than taking melatonin on your own judgement, talk to a specialist. If the problem is falling asleep and waking up often, read insomnia: what helps as well.

Myth 1: Morning people are more successful and smarter. In Korea the phrase “morning person” spread with the October 2003 Korean edition of a book by the Japanese author Hiroshi Saisho, and in March 2004 a weekly magazine reported that “morning people are all the rage in Korea” (Weekly Kyunghyang, 2004). Yet in a meta-analysis of 30 studies, morningness and intelligence were not linked overall (Ujma & Scherrer, 2021), and the link with grades was small (Tonetti et al., 2015). A questionnaire study found that morning people rated themselves as more proactive, but it cannot tell whether getting up early made them so (Randler, 2009). While writing this guide, we found no study testing whether waking at 5 a.m. or a “miracle morning” routine leads to success.

Myth 2: Night owls are lazy. Being a night owl is mainly a difference in how late the body clock locks on to the day (Roenneberg et al., 2019), and in twin studies close to half of the differences between people were linked with genes (Koskenvuo et al., 2007; Hur, 2007). The link with conscientiousness is only a tendency across people (Lipnevich et al., 2017), and some researchers suggest that school timetables built around early mornings may put evening-oriented students at a disadvantage (Preckel et al., 2013).

Myth 3: Anyone can become a morning person if they set their mind to it. Light and daily schedules can bring your timing forward by about 1 to 2 hours (Wright et al., 2013; Facer-Childs et al., 2019), but it tends to slip back when you stop (Roenneberg et al., 2019). In Finnish men surveyed 23 years apart, many moved towards the morning as they aged, but very few went from clearly evening to clearly morning (Fischer et al., 2017). A more realistic aim than becoming “a true morning person” is to narrow the gap between your schedule and your body clock.

Myth 4: Sleeping in at the weekend makes up for short sleep in the week. In an experiment that randomly assigned 36 healthy young adults, the 14 who could sleep only 5 hours a night for 5 weekdays and as long as they liked for 2 weekend days slept about 1.1 hours more in total over the weekend than at baseline, but when short sleep resumed their body clocks shifted later and their after-dinner eating and body weight went up. The researchers concluded that weekend recovery sleep is not an effective way to prevent the metabolic problems of repeated short sleep (Depner et al., 2019). On the other hand, in an observational study following 43,880 Swedish adults for 13 years, among those under 65, people who slept little on weekdays but long at weekends had the same mortality as those who consistently slept 6 to 7 hours a day. But sleep was asked about only once at the start, so the authors could not be sure this was really “catching up” (Åkerstedt et al., 2019). The researchers behind the MCTQ see the root of the problem in the constraints of the working week rather than in weekend lie-ins (Roenneberg et al., 2019). If you have insomnia, the advice is to get up at your usual time rather than sleep in (insomnia: what helps).

Myth 5: There are four chronotypes: lion, bear, wolf and dolphin. The four-animal scheme is said to have spread from a popular book published in 2016, but while writing this guide we found no scientific study testing it. Research sees chronotype not as a handful of animal types but as a position on a continuum from morning to evening, with most people somewhere in the middle (Roenneberg et al., 2007; Fischer et al., 2017).

Q. Where can I take a morning or evening type test? A. Research uses questionnaires such as the MEQ and the MCTQ, but even in the same people, the share of each type changed a lot depending on where the cut-offs were drawn (Taillard et al., 2004). These questionnaires are not diagnostic tools, and mind.io has no morning or evening type test. Just as the MCTQ asks about actual sleep times on free days (Roenneberg et al., 2003), noting when you fall asleep and wake up on days without an alarm can give you an idea of your rhythm. But if you were short of sleep during the week, catch-up sleep may make you wake later on free days, which is why the MCTQ corrects for sleep debt (Roenneberg et al., 2019).

Q. Can a gene test tell me whether I am a morning person? A. No. Even in the study that found 351 genetic loci linked with morningness, the 5% with the most morningness variants slept on average only about 25 minutes earlier than the 5% with the fewest (Jones et al., 2019). That is not enough to tell one person’s type.

Q. Will taking melatonin make me a morning person? A. Talk to a sleep specialist rather than taking it on your own judgement. Specialists sometimes use timed melatonin for delayed sleep-wake phase disorder (Auger et al., 2015), but insomnia: what helps explains that it is not recommended for treating insomnia. This guide does not cover doses or timing.

Q. My teenager goes to bed very late. Is something wrong? A. Comparing age groups, the late teens are when chronotype is latest (Roenneberg et al., 2019), so to some extent this is a common change. But if they have struggled to fall asleep and wake up at the desired times for more than 3 months and daily life is suffering, consider talking to a sleep specialist (Futenma et al., 2023).

Q. Can the Big Five test tell me whether I am a morning person? A. No. Conscientiousness was the trait most linked with morningness, but all five traits together explained only part of the differences between morning and evening types (Lipnevich et al., 2017). The Big Five personality test looks at personality traits.

References: Roenneberg, T., Pilz, L. K., Zerbini, G., & Winnebeck, E. C. (2019), Chronotype and social jetlag: A (self-) critical review, Biology, 8(3), 54, https://doi.org/10.3390/biology8030054; Ministry of Health, Labour and Welfare (2025), Circadian rhythm sleep-wake disorders, by Kazuo Mishima, e-Health Net [in Japanese], https://kennet.mhlw.go.jp/information/information/heart/k-02-006.html; Ministry of Health, Labour and Welfare (n.d.), Delayed (advanced) sleep phase syndrome, e-Health Net glossary [in Japanese], https://kennet.mhlw.go.jp/information/information/dictionary/heart/yk-028.html; Roenneberg, T., Kuehnle, T., Juda, M., Kantermann, T., Allebrandt, K., Gordijn, M., & Merrow, M. (2007), Epidemiology of the human circadian clock, Sleep Medicine Reviews, 11(6), 429-438, https://doi.org/10.1016/j.smrv.2007.07.005; Wittmann, M., Dinich, J., Merrow, M., & Roenneberg, T. (2006), Social jetlag: Misalignment of biological and social time, Chronobiology International, 23(1-2), 497-509, https://doi.org/10.1080/07420520500545979; Fischer, D., Lombardi, D. A., Marucci-Wellman, H., & Roenneberg, T. (2017), Chronotypes in the US - Influence of age and sex, PLOS ONE, 12(6), e0178782, https://doi.org/10.1371/journal.pone.0178782; Horne, J. A., & Östberg, O. (1976), A self-assessment questionnaire to determine morningness-eveningness in human circadian rhythms, International Journal of Chronobiology, 4(2), 97-110, PMID 1027738, https://pubmed.ncbi.nlm.nih.gov/1027738/; Roenneberg, T., Wirz-Justice, A., & Merrow, M. (2003), Life between clocks: Daily temporal patterns of human chronotypes, Journal of Biological Rhythms, 18(1), 80-90, https://doi.org/10.1177/0748730402239679; Taillard, J., Philip, P., Chastang, J.-F., & Bioulac, B. (2004), Validation of Horne and Ostberg morningness-eveningness questionnaire in a middle-aged population of French workers, Journal of Biological Rhythms, 19(1), 76-86, https://doi.org/10.1177/0748730403259849; Paine, S.-J., Gander, P. H., & Travier, N. (2006), The epidemiology of morningness/eveningness: Influence of age, gender, ethnicity, and socioeconomic factors in adults (30-49 years), Journal of Biological Rhythms, 21(1), 68-76, https://doi.org/10.1177/0748730405283154; 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), 156-162, https://doi.org/10.1111/j.1365-2869.2007.00580.x; Hur, Y.-M. (2007), Stability of genetic influence on morningness-eveningness: A cross-sectional examination of South Korean twins from preadolescence to young adulthood, Journal of Sleep Research, 16(1), 17-23, https://doi.org/10.1111/j.1365-2869.2007.00562.x; Vink, J. M., Groot, A. S., Kerkhof, G. A., & Boomsma, D. I. (2001), Genetic analysis of morningness and eveningness, Chronobiology International, 18(5), 809-822, https://doi.org/10.1081/cbi-100107516; Toomey, R., Panizzon, M. S., Kremen, W. S., Franz, C. E., & Lyons, M. J. (2015), A twin-study of genetic contributions to morningness-eveningness and depression, Chronobiology International, 32(3), 303-309, https://doi.org/10.3109/07420528.2014.971366; Jones, S. E., Lane, J. M., Wood, A. R., van Hees, V. T., Tyrrell, J., Beaumont, R. N., et al. (2019), Genome-wide association analyses of chronotype in 697,828 individuals provides insights into circadian rhythms, Nature Communications, 10(1), 343, https://doi.org/10.1038/s41467-018-08259-7; Roenneberg, T., Kuehnle, T., Pramstaller, P. P., Ricken, J., Havel, M., Guth, A., & Merrow, M. (2004), A marker for the end of adolescence, Current Biology, 14(24), R1038-R1039, https://doi.org/10.1016/j.cub.2004.11.039 [as described in Fischer et al., 2017]; Randler, C., & Engelke, J. (2019), Gender differences in chronotype diminish with age: A meta-analysis based on morningness/chronotype questionnaires, Chronobiology International, 36(7), 888-905, https://doi.org/10.1080/07420528.2019.1585867; Wright, K. P., Jr., McHill, A. W., Birks, B. R., Griffin, B. R., Rusterholz, T., & Chinoy, E. D. (2013), Entrainment of the human circadian clock to the natural light-dark cycle, Current Biology, 23(16), 1554-1558, https://doi.org/10.1016/j.cub.2013.06.039; Stothard, E. R., McHill, A. W., Depner, C. M., Birks, B. R., Moehlman, T. M., Ritchie, H. K., et al. (2017), Circadian entrainment to the natural light-dark cycle across seasons and the weekend, Current Biology, 27(4), 508-513, https://doi.org/10.1016/j.cub.2016.12.041; Facer-Childs, E. R., Middleton, B., Skene, D. J., & Bagshaw, A. P. (2019), Resetting the late timing of ‘night owls’ has a positive impact on mental health and performance, Sleep Medicine, 60, 236-247, https://doi.org/10.1016/j.sleep.2019.05.001 [participant number and instructions from the University of Birmingham news release, 2019: https://www.birmingham.ac.uk/news-archive/2019/night-owls-can-retrain-their-body-clocks-to-improve-mental-well-being-and-performance]; Vetter, C., Fischer, D., Matera, J. L., & Roenneberg, T. (2015), Aligning work and circadian time in shift workers improves sleep and reduces circadian disruption, Current Biology, 25(7), 907-911, https://doi.org/10.1016/j.cub.2015.01.064; Au, J., & Reece, J. (2017), The relationship between chronotype and depressive symptoms: A meta-analysis, Journal of Affective Disorders, 218, 93-104, https://doi.org/10.1016/j.jad.2017.04.021; Norbury, R. (2021), Diurnal preference and depressive symptomatology: A meta-analysis, Scientific Reports, 11(1), 12003, https://doi.org/10.1038/s41598-021-91205-3; Daghlas, I., Lane, J. M., Saxena, R., & Vetter, C. (2021), Genetically proxied diurnal preference, sleep timing, and risk of major depressive disorder, JAMA Psychiatry, 78(8), 903-910, https://doi.org/10.1001/jamapsychiatry.2021.0959; Jeong, S. J., Kim, C. G., & Son, J. H. (2025), The effects of sleep patterns and chronotype on depressive symptoms in adolescents: A cross-sectional study using data from the 19th Korea Youth Risk Behavior Web-based Survey, Journal of Korean Biological Nursing Science, 27(4), 541-553, https://doi.org/10.7586/jkbns.25.043; Knutson, K. L., & von Schantz, M. (2018), Associations between chronotype, morbidity and mortality in the UK Biobank cohort, Chronobiology International, 35(8), 1045-1053, https://doi.org/10.1080/07420528.2018.1454458; Roenneberg, T., Allebrandt, K. V., Merrow, M., & Vetter, C. (2012), Social jetlag and obesity, Current Biology, 22(10), 939-943, https://doi.org/10.1016/j.cub.2012.03.038; Tsaousis, I. (2010), Circadian preferences and personality traits: A meta-analysis, European Journal of Personality, 24(4), 356-373, https://doi.org/10.1002/per.754; Lipnevich, A. A., Credè, M., Hahn, E., Spinath, F. M., Roberts, R. D., & Preckel, F. (2017), How distinctive are morningness and eveningness from the Big Five factors of personality? A meta-analytic investigation, Journal of Personality and Social Psychology, 112(3), 491-509, https://doi.org/10.1037/pspp0000099; Ujma, P. P., & Scherrer, V. (2021), Circadian preference and intelligence - an updated meta-analysis, Chronobiology International, 38(8), 1215-1229, https://doi.org/10.1080/07420528.2021.1926473; Tonetti, L., Natale, V., & Randler, C. (2015), Association between circadian preference and academic achievement: A systematic review and meta-analysis, Chronobiology International, 32(6), 792-801, https://doi.org/10.3109/07420528.2015.1049271; Preckel, F., Lipnevich, A. A., Boehme, K., Brandner, L., Georgi, K., Könen, T., Mursin, K., & Roberts, R. D. (2013), Morningness-eveningness and educational outcomes: The lark has an advantage over the owl at high school, British Journal of Educational Psychology, 83(1), 114-134, https://doi.org/10.1111/j.2044-8279.2011.02059.x; Shimura, A., Sakai, H., & Inoue, T. (2022), Paradoxical association between chronotype and academic achievement: Eveningness reduces academic achievement through sleep disturbance and daytime sleepiness, Sleep and Biological Rhythms, 20(3), 353-359, https://doi.org/10.1007/s41105-022-00375-8; May, C. P., Hasher, L., & Healey, K. (2023), For whom (and when) the time bell tolls: Chronotypes and the synchrony effect, Perspectives on Psychological Science, 18(6), 1520-1536, https://doi.org/10.1177/17456916231178553; Schmidt, C., Collette, F., Cajochen, C., & Peigneux, P. (2007), A time to think: Circadian rhythms in human cognition, Cognitive Neuropsychology, 24(7), 755-789, https://doi.org/10.1080/02643290701754158; American Academy of Pediatrics, Adolescent Sleep Working Group, Committee on Adolescence, & Council on School Health (2014), School start times for adolescents, Pediatrics, 134(3), 642-649, https://doi.org/10.1542/peds.2014-1697; Marx, R., Tanner-Smith, E. E., Davison, C. M., Ufholz, L.-A., Freeman, J., Shankar, R., Newton, L., Brown, R. S., Parpia, A. S., Cozma, I., & Hendrikx, S. (2017), Later school start times for supporting the education, health, and well-being of high school students, Cochrane Database of Systematic Reviews, 2017(7), CD009467, https://doi.org/10.1002/14651858.CD009467.pub2; Jung, J., & Fenelon, A. (2024), Do later school start times improve adolescents’ sleep and substance use? A quasi-experimental study, Preventive Medicine, 185, 108028, https://doi.org/10.1016/j.ypmed.2024.108028; Futenma, K., Takaesu, Y., Komada, Y., Shimura, A., Okajima, I., Matsui, K., Tanioka, K., & Inoue, Y. (2023), Delayed sleep-wake phase disorder and its related sleep behaviors in the young generation, Frontiers in Psychiatry, 14, 1174719, https://doi.org/10.3389/fpsyt.2023.1174719; Sivertsen, B., Pallesen, S., Stormark, K. M., Bøe, T., Lundervold, A. J., & Hysing, M. (2013), Delayed sleep phase syndrome in adolescents: Prevalence and correlates in a large population based study, BMC Public Health, 13, 1163, https://doi.org/10.1186/1471-2458-13-1163; Sivertsen, B., Harvey, A. G., Gradisar, M., Pallesen, S., & Hysing, M. (2021), Delayed sleep-wake phase disorder in young adults: Prevalence and correlates from a national survey of Norwegian university students, Sleep Medicine, 77, 184-191, https://doi.org/10.1016/j.sleep.2020.09.028; Gradisar, M., & Crowley, S. J. (2013), Delayed sleep phase disorder in youth, Current Opinion in Psychiatry, 26(6), 580-585, https://doi.org/10.1097/YCO.0b013e328365a1d4; Auger, R. R., Burgess, H. J., Emens, J. S., Deriy, L. V., Thomas, S. M., & Sharkey, K. M. (2015), Clinical practice guideline for the treatment of intrinsic circadian rhythm sleep-wake disorders: Advanced sleep-wake phase disorder (ASWPD), delayed sleep-wake phase disorder (DSWPD), non-24-hour sleep-wake rhythm disorder (N24SWD), and irregular sleep-wake rhythm disorder (ISWRD). An update for 2015, Journal of Clinical Sleep Medicine, 11(10), 1199-1236, https://doi.org/10.5664/jcsm.5100; Weekly Kyunghyang (2004), “Easy-going people” do well, by Lee Su-ji, 11 March 2004 [in Korean], https://weekly.khan.co.kr/article/6704; Randler, C. (2009), Proactive people are morning people, Journal of Applied Social Psychology, 39(12), 2787-2797, https://doi.org/10.1111/j.1559-1816.2009.00549.x; Depner, C. M., Melanson, E. L., Eckel, R. H., Snell-Bergeon, J. K., Perreault, L., Bergman, B. C., et al. (2019), Ad libitum weekend recovery sleep fails to prevent metabolic dysregulation during a repeating pattern of insufficient sleep and weekend recovery sleep, Current Biology, 29(6), 957-967.e4, https://doi.org/10.1016/j.cub.2019.01.069; Åkerstedt, T., Ghilotti, F., Grotta, A., Zhao, H., Adami, H.-O., Trolle-Lagerros, Y., & Bellocco, R. (2019), Sleep duration and mortality - Does weekend sleep matter?, Journal of Sleep Research, 28(1), e12712, https://doi.org/10.1111/jsr.12712