Published : 21 Oct 2026, 05:31 PM
Updated : 21 Oct 2026, 05:31 PM
Scientists have identified a new subtype of depression tied to the body’s internal clock, a finding that could explain why standard antidepressant treatments fail a significant number of patients and point the way towards more targeted therapies, The Independent reports.
The condition, termed “circadian depression”, affects roughly one in four people among those with severe, recurrent or persistent depression, according to a study published in the British Journal of Psychiatry by researchers from the University of Sydney.
Circadian depression is characterised by a seasonal pattern of illness, sleeping longer during depressive episodes, waking feeling exhausted, and a tendency towards being a night owl. People with the subtype are more likely to be female, develop depression earlier in life, and experience more severe symptoms.
They also carry a higher genetic risk for inflammatory and cardiovascular conditions, as well as neurological disorders including ADHD, anxiety, and bipolar disorder.
Crucially, the condition responds poorly to selective serotonin reuptake inhibitors (SSRIs) -- the most commonly prescribed antidepressants -- but may respond better to therapies that target circadian rhythms, such as melatonin, melatonin-targeted antidepressants, and light therapy.
The research drew on genetic analysis of more than 10,000 Australians with depression. Of those, 2,604 -- nearly 25 per cent -- were identified as having the circadian subtype.
“Depression is not a uniform illness and understanding the characteristics of different subtypes means that we can give people earlier access to treatments that will be most effective for them,” study author Emiliana Tonini said in the British Journal of Psychiatry paper.
Co-author Jacob Crouse said clinicians should consider circadian-targeted treatments earlier for patients who fit the profile. “For those who find that antidepressants are not working for them, it is important to speak to your clinician for advice about your individual circumstances,” he added, as quoted by The Independent.
The researchers called for larger, more diverse follow-up studies to validate the findings.
