Australian study identifies 'circadian' subtype of depression

Nearly one in four people in a large Australian study of severe, recurrent or persistent depression shared a recognisable pattern: a seasonal rhythm of illness, sleeping longer when depressed, waking feeling very tired and being a night owl. University of Sydney researchers have now given that pattern a name, 'circadian depression', and say it is a distinct subtype of the illness.

Antidepressants of the SSRI family (selective serotonin reuptake inhibitors) are usually the first option for treating depression. For people with the circadian subtype, the study finds they are less effective, and come with more side effects. The reason lies in the body's 24-hour internal clock, which regulates the sleep-wake cycle. Because this subtype is linked to the clock's own brain circuits, treatments that target those circuits, such as melatonin, melatonin-targeted antidepressants and light therapy, are more responsive in these patients.

The study used clinical and genetic data from more than 10,000 people with depression, drawn from the nationwide Australian Genetics of Depression Study. Of them, 2,604 participants were classed as having circadian depression. Those patients were more likely to have their depression begin earlier in life, and they had more severe symptoms, including mania and higher levels of suicidal thoughts. They also carried a higher genetic risk for inflammatory and cardiovascular health problems and for major neurodevelopmental disorders, including ADHD. The study identifies these as genetic correlations; it does not establish that the subtype causes them.

For young people, the stakes are immediate. Ian Hickie, co-director of the Brain and Mind Centre and a clinical psychiatrist, says that failing to recognise circadian depression in young people often led to early treatment failures and more side effects. "Most young people will be offered SSRIs first, but for at least a quarter there are better circadian-focused medical and behavioral options," he said. Co-senior author Dr. Jacob Crouse said clinicians 'should reach for these much earlier' in patients with this presentation. The study sets no date for that change in practice, and it leaves the decision with clinicians and with patients whose current antidepressants are not working, who are advised to speak to their clinician about their own circumstances.

The findings rest on one national cohort, and the study does not address whether other populations show the same proportion of the subtype. Still, the authors' point is simple: depression is not a single illness, and a clearer map of its subtypes lets clinicians offer the treatment most likely to work, sooner.