# New Depression Subtype Identified, Linked to Circadian Rhythms

> Australian researchers have identified 'circadian depression,' a distinct subtype affecting nearly one-quarter of individuals with severe depression, which responds differently to treatment.

By PrimaryBrief newsroom · Published 2026-10-24T01:56:13.983529Z
Canonical: https://primarybrief.com/article/new-depression-subtype-identified-linked-to-circadian-rhythms-63b75337

University of Sydney researchers announced the identification of a distinct "circadian depression" subtype on October 21, 2026, affecting nearly one-quarter of people with severe, recurrent, or persistent depression [3][6][10][12]. This subtype is characterized by specific sleep patterns and genetic markers, responding poorly to traditional antidepressants but well to therapies targeting the body's internal clock [2][6][9].

This discovery could lead to more personalized and effective treatments for many suffering from depression, preventing prolonged ineffective therapies and reducing side effects [3][6]. Understanding these biological differences means clinicians can provide earlier access to the most effective interventions, especially for young people [3][6].

## What the records show
A study published in the British Journal of Psychiatry on October 21, 2026, analyzed clinical and genetic data from 11,380 Australians diagnosed with severe, recurrent, or persistent depression [3][10][16]. Researchers identified 2,604 participants, representing approximately 23% of the total, as having a circadian depression subtype [12][16][17]. Individuals were classified if they met at least three out of six circadian-associated features: a seasonal pattern of illness, delayed sleep midpoint, social jetlag, an evening chronotype, hypersomnia, and morning tiredness [2][10].

This subtype presents with earlier symptom onset, around 18 years old, compared to 23 years for other forms of depression [15][16]. Participants with circadian depression also experienced more severe clinical features, including mania, psychosis, and higher levels of suicidal thoughts, alongside weight gain during depressive episodes [14][15][16]. Genetic analysis indicated a higher genetic risk for conditions such as attention-deficit/hyperactivity disorder (ADHD), major depression, bipolar disorder, and metabolic markers like high body mass index and insulin resistance [13][15][16]. Dr. Emiliana Tonini, from the Brain and Mind Centre, stated that this suggests "circadian depression is more than just 'depression with bad sleep.' It involves a mix of metabolic, inflammatory, circadian, and mood-related processes" [13].

Traditional first-line antidepressants, including selective serotonin reuptake inhibitors (SSRIs) and serotonin-norepinephrine reuptake inhibitors (SNRIs), proved less effective and caused more side effects for individuals with this subtype [9][14][16]. Conversely, these individuals showed greater responsiveness to behavioral and medical therapies aimed at the body's circadian rhythms, such as melatonin, melatonin-targeted antidepressants, and light therapy [6][9][14]. Jacob Crouse noted that treatments targeting the circadian rhythm "may be much more relevant, and clinicians should reach for these much earlier in people with this presentation" [14].

## The timeline
The initial research identifying the circadian depression subtype was released as a preprint on medRxiv on February 23, 2026 [4][5][17]. The full study, titled "Clinical and genetic correlates of a circadian subtype of depression in the Australian Genetics of Depression Study," was subsequently published in the British Journal of Psychiatry on October 21, 2026 [1][3][16].

## What changed and for whom
The identification of circadian depression provides a new framework for understanding and treating a significant portion of individuals with severe depression [3][6]. For the approximately one-quarter of individuals who fit this subtype, current treatment protocols, which often begin with SSRIs, may be less effective and cause more adverse reactions [6][9][14]. Professor Ian Hickie emphasized the need to ensure young people receive appropriate treatment early, noting that for at least a quarter, "there are better circadian-focused medical and behavioural options" than SSRIs [6]. The findings advocate for clinicians to consider therapies like light therapy and melatonin earlier in the treatment process for these patients [14].

## What the subject has said publicly
Dr. Emiliana Tonini, a lead researcher from the University of Sydney's Brain and Mind Centre, said, "Depression is not a uniform illness and understanding the characteristics of different subtypes mean that we can give people earlier access to treatments that will be most effective for them" [3][6]. Professor Ian Hickie, also from the Brain and Mind Centre, highlighted that not recognizing circadian depression frequently leads to early treatment failures and increased side effects in young patients [6].

Eduard Vieta, Director of the Institute of Neurosciences at Hospital Clínic and Professor of Psychiatry at the University of Barcelona, offered a nuanced perspective [1]. He suggested that describing it as a "new subtype" might be an overstatement, pointing out that the DSM already describes patients with a 'seasonal pattern' and that 'atypical depression' shares many described circadian features [1]. The study authors, however, maintain that their findings support the "biological validity and clinical utility of a circadian subtype of depression as a clinical profile" [1][2].

## What we could not confirm
The research details how clinicians should precisely identify these specific circadian-associated features in routine practice.

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50. PPP126: Treatment resistant depression with Professor Ian Hickie — Purple Pen Podcast (purplepenpodcast.com) — https://purplepenpodcast.com/blog/2021/12/19/ppp126-treatment-resistant-depression-with-professor-ian-hickie