Night Owl or Early Bird? McGill Study Finds Sleep Categories Aren't That Simple

McGill Researchers Uncover Five Distinct Chronotype Subtypes

The latest research from McGill University has challenged the long-held binary view of sleep preferences, revealing that human chronotypes—our natural inclinations toward being night owls or early birds—are far more nuanced. Led by PhD student Le Zhou and senior author Associate Professor Danilo Bzdok from McGill's Department of Biomedical Engineering, the study analyzed data from over 27,000 adults in the UK Biobank. Published today in Nature Communications, the findings identify five biologically distinct sleep-wake profiles, each linked to unique brain networks, behaviors, and health outcomes.

Chronotype, formally defined as the individual preference for the timing of sleep and wakefulness within a 24-hour cycle, has traditionally been simplified into morning types (early birds) and evening types (night owls). However, this McGill sleep chronotype study demonstrates that these categories encompass heterogeneous subgroups influenced by genetic, environmental, and lifestyle factors. By integrating brain imaging data—such as gray matter volume, white matter integrity, and functional connectivity—with extensive behavioral questionnaires and medical records, researchers used advanced AI-driven pattern learning to delineate these subtypes.

This breakthrough not only explains inconsistencies in prior research linking evening chronotypes to poorer health but also opens doors to personalized interventions, particularly relevant for university students and academics in Canada where irregular schedules are common.

Understanding Chronotype: Beyond the Basics

A chronotype reflects the phase of an individual's endogenous circadian rhythm, determining when they naturally feel alert or sleepy. The suprachiasmatic nucleus in the hypothalamus acts as the body's master clock, synchronized by light exposure and other zeitgebers (time-givers). Genetic variations in clock genes like PER2 and CLOCK contribute to chronotype differences, with heritability estimated at 40-50%.

In Canada, surveys of university students reveal a skew toward evening chronotypes. For instance, a study of over 2,300 Canadian undergraduates found evening types reported higher rates of depression and anxiety, poorer sleep quality, and lower social support compared to morning types. Prevalence data indicates about 20-30% of young adults identify as evening chronotypes, rising in student populations due to late-night studying and social activities.

McGill's prior research, including work on shift workers, showed that chronotype mismatches with schedules exacerbate sleep deficits. This new study builds on that by incorporating neuroimaging, providing a neurobiological foundation.

The Innovative Methodology of the McGill Study

The research employed supervised partial least squares (PLS) regression, a multivariate statistical technique ideal for high-dimensional data integration. Three brain imaging modalities were analyzed: structural MRI for gray matter volume across 139 regions, diffusion MRI for white matter fractional anisotropy in 48 tracts, and resting-state fMRI for 210 functional connections from 21 independent components.

Data from 27,030 UK Biobank participants (aged 40-70, 54% female) underwent preprocessing to regress out confounds like age, sex, BMI, and sleep duration. Phenome-wide association studies (PheWAS) linked subtypes to 977 behaviors, 1,396 diagnoses, and 133 medication categories. Validation occurred in 10,550 US adolescents from the ABCD Study, confirming replicability across ages and populations.

This rigorous, AI-augmented approach, supported by computational power from McGill's McConnell Brain Imaging Centre, marks a shift from subjective questionnaires to objective multimodal profiling.Read the full paper here.

Profiling the Five Chronotype Subtypes

The study pinpointed five latent brain subtypes within traditional chronotypes:

• Subtype 1 (Night Owl - Cognitive Strengths): Prevalent in younger adults, associated with faster reaction times, better puzzle-solving, but higher risk-taking, irritability, and substance use. Brain features: increased limbic and frontal gray matter, enhanced somatomotor connectivity.
• Subtype 2 (Night Owl - Health Risks): Linked to depression, smoking, hypertension, diabetes, and cardiovascular disease. Reduced white matter integrity across tracts; altered temporal-basal ganglia connectivity.
• Subtype 3 (Early Bird - Healthiest): Fewest health issues, non-smokers, earlier risers, more TV viewing. Opposite brain patterns to Subtype 1, with stronger frontal and parahippocampal volumes.
• Subtype 4 (Female-Biased Early Bird): Higher depression rates, menstruation disorders, lower testosterone. Subcortical increases (hippocampus, amygdala), reduced white matter integrity.
• Subtype 5 (Male-Biased Night Owl): Hair balding, prostate issues, higher testosterone, risk-taking. Basal ganglia and visual area expansions, mixed white matter changes.

These profiles explain why not all night owls face identical risks—vulnerabilities are subtype-specific.

Photo by Igor Savelev on Unsplash

Brain Networks: The Neural Underpinnings

Each subtype exhibits distinct resting-state networks. For example, Subtype 1 shows heightened frontoparietal attention and somatosensory-motor links, supporting cognitive advantages but emotional challenges. Subtype 2's widespread white matter reductions suggest disrupted communication, correlating with metabolic disorders.

Sex differences emerge prominently: Subtypes 4 and 5 align with hormonal profiles, with females overrepresented in depression-linked morningness. Age gradients reverse between cohorts—night owl subtypes dominate youth in adults but shift in adolescents—indicating developmental trajectories.

For Canadian higher education, where neuroimaging resources like McGill's are advancing knowledge, these insights could inform student wellness programs.Explore higher ed career advice for balancing academic demands.

Health and Behavioral Implications

Health risks vary: Subtypes 2 and 4 elevate depression odds, while 2 and 5 heighten cardiovascular threats. Behaviorally, evening subtypes link to substance use and sedentary habits, though Subtype 1 excels cognitively—potentially advantageous for late-night research.

In Canadian universities, evening chronotypes comprise up to 25% of students, correlating with 20-30% higher mental health issues per Memorial University data. Social jetlag—misalignment between biological and social clocks—exacerbates this, especially during exam periods.

Relevance to Canadian University Students and Faculty

McGill University, a leader in neuroscience, highlights how chronotype impacts higher education. Canadian studies show evening-type students at Memorial and other institutions experience poorer sleep (Pittsburgh Sleep Quality Index scores >5 in 60%) and higher anxiety.

Resident physicians at McGill-affiliated Douglas Mental Health Institute report 51% sleep disturbances, with severe social jetlag in early training years. Professors with irregular seminar schedules may benefit from subtype awareness for productivity.

Institutions could adapt: flexible class times, chronotype assessments for advising. Rate My Professor reviews often note profs' energy levels, tying to chronotypes.

McGill's Legacy in Sleep and Circadian Research

McGill has pioneered sleep science, from Diane Boivin's shift-work chronotype studies to Marie-Claude Geoffroy's resident physician analyses. Earlier work linked extreme eveningness to bipolar risks and ADHD.

This study, funded by Brain Canada and CIHR, leverages Mila's AI expertise. It positions McGill as a hub for chronobiology, attracting higher ed jobs in neuroscience.

Photo by Bermix Studio on Unsplash

Future Directions and Actionable Insights

Researchers plan genetic analyses to trace subtype origins. Personalized chronotherapeutics—light therapy for Subtype 2, schedule tweaks for others—loom large.

• Track your chronotype via Munich Chronotype Questionnaire.
• Maintain consistent sleep hygiene despite academic demands.
• Seek wellness resources; align shifts with biology.
• For faculty, consider career advice on work-life balance.

Canadian universities should integrate these findings into health services.

Conclusion: Embracing Sleep Diversity

The McGill sleep chronotype study redefines rest, urging tailored strategies for thriving in academia. Whether subtype 1's cognitive whiz or 3's steady performer, understanding your profile empowers better health and performance. Explore opportunities at university jobs, higher ed jobs, Rate My Professor, and higher ed career advice while prioritizing sleep.