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A study has identified a new subtype of depression called circadian depression. As the name suggests, circadian depression is linked to the body's 24-hour internal clock. The study published in The British Journal of Psychiatry investigated whether major depressive disorder can be meaningfully categorised into a distinct "circadian subtype" based on biological, clinical, and genetic features. Researchers conducted a clinical and genetic analysis of more than 10,000 people with depression using data from the Australian Genetics of Depression Study.
Many people with major depressive disorder do not respond to standard antidepressants, which treat this condition as if it were the same for everyone. This study aimed to explore a type of depression called "circadian subtype." This type is linked to problems with your body clock, such as irregular sleep patterns, changes with the seasons, and sleep issues. The goal was to see how this circadian subtype differs from other forms of depression in terms of symptoms and genetic factors.
"Our research has identified circadian depression as a new subtype, affecting around a quarter of people in our study, including those with severe, recurrent or persistent depression," first author Emiliana Tonini from the Brain and Mind Centre at the University of Sydney, said.
Symptoms of circadian depression
People with this subtype typically present with a specific cluster of sleep-wake and seasonal symptoms:
- Extreme morning fatigue
- Sleeping for significantly longer periods during episodes
- Night owl tendencies
- Delayed sleep midpoint
- Social jet lag
- A seasonal pattern of illness
"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," Tonini said.
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Compared to individuals with the non-circadian subtype of depression, those with circadian depression tend to be:
1. Younger on average, with a mean age of around 18.7 years, compared to 23.2 years for the non-circadian subtype
2. Predominantly female, making up approximately 80% of cases
3. Experiencing an earlier age of onset
4. Showing greater clinical severity, which includes higher rates of:
- Hypomanic or manic-like features (similar to bipolar symptoms)
- Psychotic-like experiences
- Suicidal thoughts and severity
- Both physical and psychological symptoms, including fatigue and somatic complaints.Differential treatment response
The study found that individuals with the circadian subtype self-reported significantly lower efficacy from common serotonin-based antidepressants, specifically selective serotonin reuptake inhibitors (SSRIs) and serotonin-norepinephrine reuptake inhibitors (SNRIs). On the other hand, they are more responsive to behavioural and medical therapies targeting the body clock's unique brain circuits, such as melatonin, melatonin-targeted antidepressants and light therapy, the researchers said. This provides a mechanistic explanation for why traditional first-line medications often fail for this specific patient profile.
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Author Dr Ian Hickie, co-director of the Brain and Mind Centre and a clinical psychiatrist, emphasised that not recognising circadian depression in young people often led to early treatment failures and more side effects.
"Most young people will be offered SSRIs first up, but for at least a quarter there are better circadian-focused medical and behavioural options to help treat and manage their condition," Hickie said.
The study concludes that circadian depression is a clinically and biologically valid subtype rather than just a collection of random sleep complaints.
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