Summary: Are you a morning lark or a night owl? The answer is written in your hair. Researchers have developed a breakthrough diagnostic test that determines a person’s chronotype (internal biological clock) using just a few hair follicles. By analyzing the activity of 17 specific genes in the hair root, the team can map an individual’s circadian rhythm without the need for hours of lab-based saliva testing.
The study of over 4,000 participants reveals that while genetics and gender play a role, lifestyle and employment status shift our internal clocks far more than previously realized.
Key Facts
- 17-Gene Signature: The test uses machine learning to analyze a pattern of 17 genes in hair follicle cells that act as the body’s “molecular gears.”
- Lifestyle Over Genetics: The data showed that employed people have internal clocks that run about 30 minutes earlier than those who are not employed, proving that social “zeitgebers” (time-givers) like work schedules significantly reshape our biology.
- The Gender Gap: Women’s internal clocks signal the start of “biological night” about six minutes earlier than men’s—a smaller difference than previously thought, but statistically significant.
- Age Migration: The study confirmed that biological rhythms shift with age; people in their mid-20s stay alert roughly one hour later than those over 50.
- Circadian Medicine: This test paves the way for “timed” healthcare, where cancer immunotherapies or blood pressure medications are administered at the exact hour they will be most effective for a specific patient.
Source: Charité
A research team at Charité – Universitätsmedizin Berlin has developed a test that can determine a person’s chronotype based on their hair roots. It is intended to lay the foundation for circadian medicine – that is, medicine that is more closely aligned with the human body’s internal clock.
Applied to approximately 4,000 people, the new method also reveals that women and men differ slightly in their biological rhythms, and that lifestyle has a greater influence than previously assumed. The results have now been published in the journal PNAS.
The clocks have changed, but the body hasn’t yet: Many people are particularly aware of their internal clock after the time change; they experience a kind of jet lag because the social time and their internal rhythm no longer align. However, the biological rhythm influences not only sleep but also metabolism and even the effects of medications.
“For example, studies show that the time of day at which certain cancer immunotherapies are administered can have a substantial impact on their effectiveness,” says Prof. Achim Kramer, head of the Division of Chronobiology at the Department of Anesthesiology and Intensive Care Medicine at Charité.
“This is likely because – like most of our body’s organs – the immune system also follows a roughly 24-hour rhythm. And that rhythm varies from person to person.”
The goal of circadian medicine is to systematically take this individual rhythm of the internal clock into account in diagnostics and therapy.
In a new Collaborative Research Center led by Achim Kramer, scientists from Charité and the Universität zu Lübeck are working to advance this field of research. Circadian medicine requires methods that allow the biological rhythm to be determined as simply as possible.
This has been difficult until now: “The standard method to date measures the ‘dark hormone’ melatonin in saliva under dim light over several hours,” explains Achim Kramer. “This can only be done in a laboratory and is too cumbersome for widespread use.”
17 genes in hair roots indicate the internal time
Achim Kramer and his team have now developed a test that determines the rhythm of the body’s internal clock using hair – or, more precisely, using cells from a few hair follicles.
“In these cells, we measure the activity of 17 genes that are part of the molecular clock or are controlled by it,” explains the chronobiologist.
“Using machine learning, this pattern can be used to calculate at what point in the daily rhythm the person is currently at. A single sample is sufficient for this.”
In the current study, the researchers demonstrated that the new test determines an individual’s circadian rhythm almost as accurately as the previous standard method.
“Hair analysis is, however, far easier to perform, which is what makes the method so valuable,” emphasizes Achim Kramer.
The team has already demonstrated that the test is suitable for widespread use: More than 4,000 people sent in hair samples from home to have their chronotype determined.
Genetic predisposition, age, gender, and lifestyle shape the internal clock
The analysis of this sample confirmed for the first time on a large scale, using biological measurements, findings that surveys had already suggested: For example, that the biological rhythm depends on age, meaning people in their mid-20s become tired on average about an hour later than those over 50. And that the internal clock signals the start of the night slightly earlier on average for the women tested than for the men.
However, at six minutes, the difference revealed by the current study is smaller than that determined in questionnaire-based studies. “We nevertheless assume that gender affects the internal clock, as sex hormones have also been shown to influence biological rhythms in other studies,” explains Achim Kramer.
Overall, a person’s chronotype is determined by several factors. “Genetic predisposition, age, gender, and lifestyle all play a role,” says the chronobiologist. “And that’s why individual people’s internal clocks can differ significantly.”
The researchers were surprised by how strongly lifestyle influences the biological rhythm: As the data show, the internal clock is active about half an hour earlier in working people than in those who are not employed.
Next step: circadian medicine
To further establish the new test, the research team is working to standardize it for routine laboratory use. This will make it even easier to apply in medical practice in the future – for example, as a basis for sleep counseling or for diagnosing irregular sleep rhythms.
Circadian medicine is also closer to becoming a reality. The test can now be used to determine whether therapies that are tailored to an individual’s internal clock are more effective or have fewer side effects than those without such timing adjustments.
About the study
The company BodyClock Technologies GmbH was spun off from Charité to market the new test; Prof. Achim Kramer is one of the shareholders and patent holders. BodyClock Technologies GmbH collected the data on the approximately 4,000 samples analyzed in the current study.
Parts of the study were conducted within the framework of the Collaborative Research Center “Foundations of Circadian Medicine” (TRR 418), which is funded by the German Research Foundation (DFG).
Key Questions Answered:
A: Surveys are subjective and often influenced by caffeine or work stress. This test measures actual gene activity. As Prof. Achim Kramer notes, your body’s “biological night” might start much earlier or later than you think it does. Knowing your true molecular time is vital for medical treatments, like knowing exactly when your immune system is most active to fight a tumor.
A: Yes. One of the study’s biggest surprises was the “lifestyle effect.” Working individuals had clocks shifted 30 minutes earlier than non-working individuals. This suggests that while we have a genetic “starting point,” our daily routines can physically rewire the expression of our clock genes.
A: Most people experience “social jet lag”—the gap between what their boss wants and what their cells want. This test allows doctors to diagnose irregular sleep rhythms and provide personalized “sleep counseling” or light therapy to help sync your social life with your biological one.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- Journal paper reviewed in full.
- Additional context added by our staff.
About this circadian rhythm research news
Author: Markus Heggen
Source: Charité
Contact: Markus Heggen – Charité
Image: The image is credited to Neuroscience News
Original Research: Closed access.
“HairTime: A noninvasive assay for estimating circadian phase from a single hair sample” by Bert Maier, Luísa K. Pilz, Selin Özcakir, Ali Rahjouei, Ashraf N. Abdo, Jan de Zeeuw, Dieter Kunz, and Achim Kramer. PNAS
DOI:10.1073/pnas.2514928123
Abstract
Circadian clocks govern daily physiological and behavioral processes and are crucial for health; disruptions can lead to various diseases.
The circadian phase of entrainment—the phase of the internal circadian clock in relation to external environmental cycles—is influenced by both genetic and environmental factors, varies between individuals, and is reflected in daily behaviors such as sleep–wake patterns, cognitive performance, and physical activity.
While circadian phase may also fluctuate within individuals, the dynamics and extent of such variation in daily life remain largely unexplored. The gold standard for circadian phase assessment, dim-light melatonin onset (DLMO), is impractical for large-scale studies, and blood-based molecular biomarkers, while promising, are limited in feasibility.
To address these challenges, we developed HairTime, a noninvasive assay that estimates circadian phase from a single daytime hair sample. Developed and evaluated in two steps—a training and a validation study—HairTime demonstrated strong predictive power compared to DLMO. Suitable for large-scale studies, it was assessed using over 4,000 samples.
Circadian phase estimations showed a normal distribution and were associated with age, sex, and notably, work schedules, with earlier timing on workdays, suggesting that societal factors can modulate internal rhythms.
Together, these findings establish HairTime as a promising tool for assessing circadian phase in research and lay the foundation for future applications in personalized chronotherapy.
