Australian study identifies circadian depression subtype affecting 23% of severe cases

The University of Sydney’s Brain and Mind Centre published a study in the British Journal of Psychiatry that identified a new depression subtype called circadian depression. According to lead researcher Emiliana Tonini, 2,604 of 11,380 participants met the criteria for the subtype. The criteria require at least three of six features: seasonal mood swings, delayed sleep timing, longer sleep when depressed, night‑owl habits, fatigue on waking, and social jetlag.

Participants with circadian depression were younger at onset, averaging 19 years, compared with 23 years for other participants. They also reported more episodes, higher suicidality, and more severe symptoms such as hypomania and psychotic‑like features. Tonini said the group had a higher genetic risk for ADHD and inflammatory markers.

The study found that standard antidepressants such as SSRIs and SNRIs were less effective for circadian depression and produced more side effects. Co‑author Jacob Crouse said treatments that target the circadian rhythm, including melatonin and light therapy, may be more relevant. Ian Hickie, co‑director of the Brain and Mind Centre, said that early recognition of circadian depression could reduce treatment failures in young people.

The research used clinical and genetic data from the Australian Genetics of Depression Study, a nationwide study of depression. The findings suggest that circadian depression is a biologically distinct phenotype that may respond better to chronobiological interventions.