How the subtype was identified

A team of Australian researchers has defined a new depression subtype, called circadian depression, which they link to disruptions in the body's internal clock. Writing in the British Journal of Psychiatry, the team drew on clinical and genetic data from 11,380 Australians whose depression was mostly severe and recurrent, and classified 23% of them - nearly a quarter - as having the circadian form.

First author Dr Emiliana Tonini of the University of Sydney's Brain and Mind Centre said the subtype was defined by at least three of six features:

• •a seasonal pattern of illness
• •delayed sleep timing
• •sleeping longer when depressed
• •being a night owl
• •waking very tired
• •social jetlag, which Tonini described as sleep during the working week differing greatly from weekend sleep

A more severe profile with distinct biology

According to Tonini, people with circadian depression carry more severe mental health symptoms, a younger age of onset, more episodes, more suicidality and more hypomanic and psychotic-like symptoms than other forms of the illness.

The genetic analysis pointed the same way: the circadian subgroup showed higher genetic risk for ADHD and mood disorders, along with a series of metabolic and inflammatory markers. Tonini said this points to a distinct biology: "circadian depression is not just depression with bad sleep, but actually there is an underlying biology".

The researchers argue the study strengthens the evidence base for a circadian subtype of depression as a biologically valid and clinically useful phenotype.

Standard antidepressants fall short

Prof Ian Hickie, a psychiatrist and co-director of the Brain and Mind Centre, said the defining concern for people in the circadian group is that the guideline-recommended therapies do not work well for them.

The study found that SSRIs and SNRIs, the typical first-line antidepressant drugs, were less effective and had more side effects for people in the circadian subtype.

Instead, the researchers suggest these patients may benefit from treatments that target the body's circadian rhythms. Hickie said chronobiological treatments are critical for this group, focused on:

• •morning light exposure
• •daylight activity
• •regularity of sleep-wake cycles and food schedules

Melatonin and light therapy are among the circadian-targeting options the researchers put forward.

Outside experts weigh in

The study drew an independent endorsement along with criticism. Prof Jayashri Kulkarni, director of the Monash Alfred Psychiatry Research Centre, who was not involved in the research, described it as a "really well-conducted study" and said the polygenic risk score ties certain types of depression to a specific genetic profile, which will matter for onward treatment. She noted, however, that the research did not account for the hormonal impacts of menopause, which she said women describe as a cause of sleep disturbance.

Prof Andrew Coogan, a behavioural neuroscientist at Maynooth University in Ireland, said the findings were not overly surprising, since major depression has long been linked to body clock changes that make people later to bed and later to rise. He criticised the lack of detail on how the criteria for the proposed subtype were developed, and said: "The results of this exploratory study are best interpreted as initial if interesting findings".

The researchers themselves flagged a limitation: participants were restricted to people of genetically inferred European ancestry, which may limit how far the results generalise, and they called for replication in more diverse samples. They hope future clinical diagnosis could enable more tailored treatments, particularly for young people seeking mental health support for the first time.