💤 Are you a night owl or an early bird? The answer is not so simple

The well-known expressions "night owl" and "early bird," long used in sleep research, do not truly capture the diversity of biological clocks in humans, a new study reveals.

Led by McGill University and published in Nature Communications, this study reveals that the two wake-sleep patterns mentioned above, called "chronotypes," actually comprise five biological subtypes in total, associated with different behaviors and health states.


A chronotype is a person's propensity to feel naturally alert or sleepy at well-defined times over a 24-hour period. Previous studies have linked evening chronotypes with poorer health, but there are many inconsistencies in these findings. The results of the new study shed light on why, the authors emphasize.

"Instead of asking whether night owls are at higher risk, perhaps we should be asking which night owls are the most vulnerable, and why," observes Le Zhou, lead author and a PhD student in McGill's Integrated Program in Neuroscience.

Night owls, early birds... and everything in between

Using AI, the research team analyzed brain imaging data, questionnaire responses, and medical records from more than 27,000 adults in the UK Biobank. To do this, they leveraged the computing resources of the McConnell Brain Imaging Centre and collaborated with scientists from the University of Montreal and the University of Oxford.

Their analysis revealed three types of night owls and two types of early birds.

On the early bird side, one group was overall the healthiest with the fewest health problems; the other showed a strong link with depression.

On the night owl side, one group performed better than others on cognitive tests, but had more difficulty regulating their emotions. Another showed a tendency towards risky behavior and cardiovascular problems, and the third group had a higher likelihood of depression, smoking, and heart disease.

"It's not just bedtime or wake-up time that defines these subtypes. They result from a complex interplay of factors related to genetics, environment, and lifestyle," explains Danilo Bzdok, an associate professor in McGill's Department of Biomedical Engineering, holder of a Canada-CIFAR AI Chair at Mila, and senior author of the paper.

Rather than making a qualitative ranking of wake-sleep types, the research team shows that the five profiles each have their own advantages and disadvantages.

A personalized approach for more restorative sleep

The diversity of wake-sleep profiles helps us understand why a sleep schedule may suit one person and not another at all; therefore, in both research and the management of sleep disorders, we must turn to personalized approaches.

"In this digital, post-pandemic era, sleep habits have never varied so much from one individual to another," notes Le Zhou. "A better understanding of this biological diversity could lead to more personalized approaches for both work schedules and support regarding sleep and mental health."

The team now hopes to analyze genetic data to determine whether chronotype subtypes are biological and therefore innate.

The study

The article "Latent brain subtypes of chronotype reveal unique behavioral and health profiles across population cohorts," by Le Zhou, Danilo Bzdok et al., was published in the journal Nature Communications.