It is 6:00 a.m. One person gets out of bed, puts on running shoes and looks suspiciously happy about the entire situation. Another looks at the alarm clock as though it has committed a crime against humanity.
Is the difference simply discipline?
Probably not.
Research suggests that when we sleep is shaped by both biology and environment.
And a twin study of young people provides a particularly interesting example.
Quick answer
Are morningness and eveningness purely discipline?
No. A twin study of young people suggests that sleep timing is associated with both genetic and shared environmental influences, and that age can change the balance.
Key takeaways
- A twin study of adolescents estimated genetic and shared environmental contributions to sleep-wake behaviours.
- Do not describe the study as a clinical chronotype diagnosis.
- The article is general research information, not medical advice or an individual prediction.
Researchers measured sleep in everyday life
The study included 495 participants aged 9–17, including:
93 pairs of identical twins
and
117 pairs of fraternal twins, along with unmatched twins.
Participants wore accelerometry devices and completed sleep diaries for up to two weeks.
Researchers examined several sleep-wake characteristics, including:
sleep onset,
wake time,
sleep midpoint,
sleep duration
and self-reported restorative sleep.
This is worth emphasizing because the study is sometimes easy to oversimplify.
It was not simply a test that labelled everyone:
MORNING PERSON
or
NIGHT OWL.
It examined measurable sleep-wake behaviours.
Genetics mattered — especially for sleep timing
Across the full sample, individual differences in sleep onset, wake time and sleep midpoint showed substantial contributions from both genetic and shared environmental factors.
For these sleep-timing measures, additive genetic factors accounted for approximately:
44–50% of the variation.
Shared family environmental factors accounted for approximately:
31–42%.
For sleep duration, the estimated genetic contribution was approximately:
62%.
For restorative sleep, it was approximately:
43%.
Once again, those percentages do not describe an individual person.
They describe variation within the studied population.
Age changed the picture
One of the most interesting findings appeared when researchers compared younger and older participants.
Among younger adolescents, shared family environment contributed substantially to several sleep-timing measures.
Among older adolescents, especially around ages 16–17, genetic contributions to timing appeared more prominent while shared environmental contribution was smaller.
That actually makes intuitive sense.
A younger child rarely has total control over when the day begins or ends.
Parents influence bedtimes.
School determines morning schedules.
Meals have times.
Activities have times.
Family routines have times.
As teenagers become older, individual biological tendencies may have more room to express themselves.
The broader lesson is important:
“Genes or environment?” is often the wrong question.
A better question is:
How do biological tendencies express themselves inside a particular environment?
Being later does not automatically mean being lazy
Modern wellness culture has developed a strange fascination with early mornings.
The “5 AM” lifestyle is sometimes presented almost as a moral achievement.
Wake early.
Train before sunrise.
Win the morning.
Win at life.
But sleep biology is more complicated than motivational slogans.
People genuinely differ in sleep timing.
At the same time, biological influence does not mean sleep timing is completely fixed.
Light exposure, age, routines, work, school and behaviour all interact with when people actually sleep.
So being naturally inclined toward later sleep is not evidence of poor character.
And having an early schedule is not proof of superior discipline.
The real problem may be the mismatch
The more useful question may not be:
“Am I a morning person or a night owl?”
It may be:
“How well does my real-life schedule fit the way I naturally tend to sleep?”
For some people, work and social schedules align reasonably well with their preferred timing.
For others, they may not.
Someone who naturally drifts later when external demands disappear may still need to wake very early every weekday.
That creates a very different lived experience from someone whose natural timing already fits the required schedule.
This is where personal data becomes interesting.
Do not present this study as a direct clinical chronotype diagnosis study.
It measured sleep timing, duration and restorative sleep.
The TwinPare perspective
Twin research reminds us that variation is not always noise.
Sometimes variation is the information.
If two people respond differently to the same schedule, the answer does not have to be that one is doing it correctly and the other incorrectly.
Their biology and environment may interact differently.
That principle sits at the heart of TwinPare Health & Fitness.
Science tells us what patterns appear across populations.
Personal longitudinal data can help us examine our own pattern.
From research to your own data
A wearable cannot perfectly determine your biological circadian system.
But long-term sleep data can still raise useful questions.
When do you tend to fall asleep when you have fewer external obligations?
When do you naturally wake?
How different are workdays from free days?
Does your sleep timing move during different training periods?
Does recovery look different when your schedule shifts?
One night's measurement is rarely very interesting.
A recurring pattern may be.
The goal is not to force everyone into the same schedule.
It is to understand the relationship between behaviour, environment and repeatedly observed patterns.
TwinPare Research — the takeaway
When young people fall asleep and wake up is influenced by both genetics and environment.
In a twin study of 495 participants aged 9–17, genetic factors contributed substantially to several measures of sleep timing, while shared family environment also played an important role — particularly among younger participants.
The study does not prove that “morning person” or “night owl” is a fixed genetic identity.
It shows something more nuanced:
our sleep timing develops through an interaction between biology and the world we live in.
Source notes
The source has been verified and editorially reviewed for this article. The limitations below show which level of conclusion the sources support.
- [ocallaghan-2021] Genetic and environmental influences on sleep-wake behaviors in adolescence. V. S. O'Callaghan et al.. Sleep Advances, 2021. Evidence type: Twin study of 495 young people aged 9–17 using accelerometry and sleep diaries to examine sleep-wake characteristics Limitation: The study examined measured sleep-wake behaviours in adolescents. It should not be presented as direct clinical chronotype diagnosis or as a genetic classification of morning and evening types. Primary source DOI
Editorial source review
This section shows how the article's key factual claims are linked to the source.
Phrasings that require caution
- Do not describe the study as a clinical chronotype diagnosis.
- Keep sleep timing, discipline and social schedule distinct.
- Present heritability as variation in the studied population.
| ID | Claim | Source support | Caution |
|---|---|---|---|
| S4 | A twin study of adolescents estimated genetic and shared environmental contributions to sleep-wake behaviours. | 2021 | Do not describe the study as a clinical chronotype diagnosis. |