A new Australian analysis of more than 11,000 people with depression suggests that irregular sleep-wake patterns may define a distinct subtype of the illness. Researchers used data from the Australian Genetics of Depression Study collected between 2016 and 2018, focusing on participants who had complete polygenic risk scores and reported at least three circadian characteristics.
Circadian depression linked to greater severity
Among the sample, 2,604 individuals – about 23% – met criteria for the circadian subtype. These participants were younger on average, with a mean age of 38, and 80% were women. The subtype was defined by at least three of six markers: social jetlag, seasonal mood shifts, delayed sleep midpoint, evening chronotype, morning fatigue, and hypersomnia.
People classified with circadian depression reported more intense psychiatric symptoms, earlier onset, and a higher number of episodes. Lifetime suicidal thoughts were noted by 65% of the group, and 31% had attempted suicide. The median number of depressive episodes was six, and the worst episode lasted around 12 weeks.
“We found that individuals with circadian depression have more severe mental health symptoms and illness scores,” said Dr. Emiliana Tonini, a postdoctoral researcher at the University of Sydney. She added that the subgroup also showed higher rates of hypomanic and psychotic-like features.
Genetic risk and treatment response
Genetic analysis revealed that the circadian group carried higher polygenic risk for several psychiatric conditions, including ADHD. Metabolic and inflammatory markers also showed raised risk scores.
When participants evaluated the effectiveness of commonly prescribed antidepressants, the circadian subtype reported lower perceived benefit from selective serotonin reuptake inhibitors (OR 0.88) and serotonin–noradrenaline reuptake inhibitors (OR 0.89). No significant differences emerged for mirtazapine or amitriptyline.
Side-effects were notably broader in the circadian group, ranging from muscle pain and blurred vision to weight gain, suicidal thoughts and reduced sexual desire. Professor Ian Hickie, co-director of Health and Policy at the Brain and Mind Centre, emphasized the importance of early identification and getting on the right track when first having symptoms.
He suggested that clinicians should assess patients’ chronotype and daily activity patterns, not just mood symptoms. Simple tools such as wearable trackers could help identify misaligned circadian rhythms and guide personalized interventions.
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In practice, aligning daily schedules with natural light exposure, waking near sunrise, staying active during daylight and limiting bright screens at night, may improve both mood and metabolic health. The researchers emphasized that these behavioral adjustments need to be consistent throughout the week, not just on weekends.
Given the distinct biological profile, the circadian subtype could warrant separate diagnostic criteria in future psychiatric manuals, moving beyond a one-size-fits-all approach to depression treatment.
Genetic insights beyond mood
The full cohort comprised 11,380 participants, 75% of whom identified as female, with an average age of 43.8 years. Only a small fraction reported a single depressive episode, indicating that recurrent illness dominated the sample.
All respondents completed self-reports on depressive symptoms, medication experiences, and circadian-related behaviours, and they were evaluated with DSM-5 criteria. Saliva samples provided polygenic risk scores for a range of mental-health and metabolic traits.
Beyond the previously noted risks, the analysis showed raised polygenic scores for metabolic and inflammatory markers. These findings suggest a broader biological footprint for the circadian subtype.
“It’s not just about sleeping better. It’s about the 24-hour clock,” Professor Hickie explained, emphasizing the central role of circadian alignment in overall health.
Clinical implications
Practical guidance includes rising close to sunrise, seeking bright daylight during mid-morning and early afternoon, and engaging in outdoor activity at sunset. Recommendations also advise against late-night exercise, bright indoor lighting and heavy meals after 9 p.m., all of which can disrupt peripheral clocks.
