The familiar labels "night owl" and "early bird," long used in sleep research, don't fully capture the diversity of human internal clocks, a new study has found.
The McGill University-led study published in Nature Communications found the two sleep-wake patterns, called chronotypes, contain a total of five distinct biological subtypes, each associated with different patterns of behaviour and health.
A chronotype is based on the parts of a 24-hour period when a person naturally feels most alert or ready to sleep. Previous research has linked late chronotypes to worse health outcomes, but results have often been inconsistent. The new findings help explain why, the authors said.
"Rather than asking whether night owls are more at risk, the better question may be which night owls are more vulnerable, and why," said lead author Le Zhou, a PhD student in McGill's Integrated Program in Neuroscience.
A broader spectrum of sleep types
Using AI, researchers combined brain imaging with questionnaires and medical records from more than 27,000 adults in the U.K. Biobank. The work drew on computational resources from the McConnell Brain Imaging Centre, and contributions from collaborators at Université de Montréal and the University of Oxford.
Their analysis revealed three types of night owls and two types of early birds.
One group of early birds had the fewest health problems overall, while the other was closely tied to depression.
As for night owls, one performed better than other groups in cognitive tests but had more emotional-regulation challenges. Another group showed a tendency toward risk-taking behaviours and cardiovascular problems, while a third was more likely to have depression, smoke and face higher risks of heart disease.
"These subtypes are not defined only by bedtime or wake-up time. They reflect a complex interaction of genetic, environmental and lifestyle factors," said senior author Danilo Bzdok, Associate Professor in McGill's Department of Biomedical Engineering and Canada CIFAR Artificial Intelligence Chair at Mila.
Rather than ranking sleep types as better or worse, the researchers highlight how risks and strengths are distributed differently across the five profiles.
Personalized approach to sleep support
A more nuanced understanding of sleep profiles can help explain why the same sleep schedule can affect people differently, shifting research and sleep support away from one-size-fits-all approaches.
"In today's digital and post-pandemic era, sleep patterns are more diverse than ever," Zhou said. "Understanding this biological diversity could eventually help inform more personalized approaches to sleep, work schedules and mental health support."
The team is now turning to genetic data to determine whether chronotype subtypes have biological roots from birth.
About the study
"Latent brain subtypes of chronotype reveal unique behavioral and health profiles across population cohorts" by Le Zhou and Danilo Bzdok et al., was published in Nature Communications.
This research was supported by the Brain Canada Foundation, the U.S. National Institutes of Health, the Canadian Institutes of Health Research, the Healthy Brains, Healthy Lives initiative, Google, the CIFAR Artificial Intelligence Chairs program and the China Scholarship Council.
