Circadian Depression Subtype Linked to Body Clock, Study Finds 

Researchers at the University of Sydney’s Brain and Mind Centre have identified a new circadian depression subtype tied to the 24-hour internal clock that regulates sleep-wake cycles. The team analysed clinical and genetic data from more than 10,000 people with depression in the Australian Genetics of Depression Study. The findings appear in the British Journal of Psychiatry. According to first author Emiliana Tonini, the subtype affects around a quarter of people in the study with severe, recurrent or persistent depression. 

People with the circadian depression subtype tend to show a seasonal pattern of illness, sleep longer when depressed, wake up very tired, and identify as night owls. The researchers found the subtype is more resistant to common serotonin-based antidepressants such as SSRIs, which also bring more side effects for these patients.  

It responds better to therapies that target the body clock’s brain circuits, including melatonin, melatonin-targeted antidepressants, and light therapy. Patients were also more likely to develop depression earlier in life and to have more severe symptoms, such as mania and higher levels of suicidal thoughts. Genetically, they carried a higher risk of inflammatory and cardiovascular problems and of neurodevelopmental disorders, including ADHD. 

Tonini said depression is not a uniform illness, and knowing its subtypes can give people earlier access to the treatments most likely to work. Co-director and psychiatrist Dr Ian Hickie warned that missing the circadian depression subtype in young people often leads to early treatment failures. Most young people are offered SSRIs first, he said, yet for at least a quarter, circadian-focused medical and behavioural options may suit them better. The study also sheds light on the genetic mechanisms behind the subtype and on why traditional SSRIs work less well for it. 

The study points toward treatment choices based on a patient’s body clock rather than a one-size-fits-all start with SSRIs. The report does not say when, or whether, clinical guidelines will change. Doctors would first need reliable ways to spot the circadian depression subtype early, and further research would need to confirm how well light therapy and melatonin-based options perform in practice.